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As you read through, please thumbs up or down to indicate if you're confident you understand each topic as you look at them and put the definition in your own words in the comments so I know how I should adjust to suit your learning needs.  I will thumbs up or thumbs down your definitions if I'm satisfied with the answered. Click on images to enlarge them.

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As you know, on the graph of a waveform the horizontal line represents a resting state, where nothing is happening, above the line is compression, where air particles squish together and below the line is rarefraction, where the particles separate. This is what we perceive as noise.

If you have two waveforms, you can plot them on the same graph. Even if the sound is the exact same for both waveforms, they are both travelling through the same air at the same time, so they affect each other. We call this interference. To work out how they affect each other, we add together both frequencies at each point along the graph.

Take the image below: at every point, the blue waveform adds with the yellow waveform to make the resulting green waveform. Where the green waveform is the same or further from the horizontal line, the blue and yellow are adding together to result in constructive interference. Where the green line is closer to the horizontal line than the blue and yellow, they add to result in destructive interference. We perceive this as being louder (higher amplitude) where they constructively interfere and quieter (lower amplitude) where they destructively interfere.

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The auditory spectrum is the range of frequencies that we can hear. In the average human, the lowest frequency that can be heard is 20Hz, and the highest frequency that can be heard is 20kHz or 20'000Hz.

This isn't set in stone, however. Some people can hear slightly higher frequencies (up to 24kHz) but it's quite uncommon. Also, as people get older, that range or spectrum shrinks, so the lowest frequency that can be heard increases from 20Hz and the highest audible frequency decreases from 20kHz.

As a side note, just because you cannot hear a frequency, that doesn't mean it cannot damage your ears. Even at 10Hz, if the amplitude is high enough it can result in hearing loss due to the air particles still physically impacting the eardrum.

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A cycle simply refers to one full compression and one full rarefraction. On the graph of the waveform below, you can see two cycles over the period of 1 second. This means that the frequency is 2 Hertz or 2 cycles per second.

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Phase is the relationship between two different waveforms. As explained in the Constructive and Destructive Interference section, two waveforms will interact with each other to create a resulting waveform.

How far apart the start of the cycles of the two initial waveforms are dictates the phase. If the start of the cycle is the same for both waveforms, then they are considered to be in phase. So if the start of the cycles are different in both waveforms, then they are out of phase.

The value used to dictate how far out of phase is degrees. Take two waveforms with the same frequency and amplitude (see image below). Perfectly in phase is 0 degrees out of phase, resulting in perfect constructive interference, but the closer to being 180 degrees out of phase they are, the more they destructively interfere to result in zero compressions and rarefractions.

A full cycle is 360 degrees, so any value higher than that will reset from 360 degrees. For example, if two waveforms of the same frequency and amplitude (see image below) are 540 degrees out of phase, then they are one cycle (360 degrees) plus another 180 degrees out of phase, resulting in perfect destructive interference, so no waveform.

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Timbre is the specific sound something makes. Every source has its own unique timbre. It is defined by the harmonic frequencies that follow the fundamental frequency.
The fundamental frequency defines the pitch, or note that it plays, but the harmonic frequencies create the identifiable differences between auditory mediums that we perceive as timbre.
Below is an example of the exact same note being played, except on the left is on an acoustic guitar and on the right is on a piano. You can see the fundamental frequency (110Hz) as the largest portion of the frequency response - that is the pitch we perceive (A2). The following harmonics are the reason we can recognise one as a guitar and the other as a piano.

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Acoustic envelope is simply the way the amplitude of a waveform moves for any sound. It can be broken up into 4 stages:

Attack: The attack is the section of the waveform where the amplitude reaches it's peak.

Decay: Decay comes immediately after the attack, and it is the fall of the waveform after reaching its peak.
Sustain: The sustain is the sustained note that you would say you "hear". In other words, it is the sustained sound that is the repeating waveform to give you pitch.
Release: This comes at the end of the sound, tapering the waveform off to zero, to end the sound.

Different sounds will have differently shaped acoustic envelopes to give them their own timbre.

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Surely you know by now that you hear through your ears. How do they work? Glad you asked.

Sound waves propagate through the air and reach the ear. The first part that they reach is the called the pinna. The pinna is all the cartilage on the outside of the ear that create the distinct "ear" shape. It's responsible for directing the sound waves into the auditory canal, and the small changes in sound by traveling through the pinna allow the brain to located where in vertical space the sound is coming from.

Once the sound goes into the auditory canal, it reaches the tympanic membrane, colloquially known as the eardrum. You can think of the way this works as the same as the skin of any drum on a drum kit, except instead of it being excited by a mallet or stick, it is excited by sound waves. The higher the amplitude of the sound wave (perceived as volume), the further the tympanic membrane oscillates. Similarly, the higher or lower the frequency of the sound wave, the more or less frequently the tympanic membrane will oscillate, causing what we perceive as pitch.

As the tympanic membrane oscillates, it transfers its energy into three bones that make up what is called the auditory ossicles. Each of the bones that makes up the auditory ossicles are sequentially called the malius, the incus, and the stapes. The role of the audtiory ossicles is to amplify the sound passed on from the tympanic membrane, turning small movements into larger ones.
The stapes oscillates like a piston, back and forth into the bony labyrinth. The bony labyrinth is filled with perilymph fluid, which continues to transfer the energy from the sound wave.

In the bony labyrinth, there is a flexible membrane called the round window, which allows the fluid to move throughout the labyrinth. If it weren't for this membrane, the fluid would stay stationary and the energy would not transfer into perceivable sound.

The section of the bony labyrinth filled with the perilymph fluid is called the cochlea. This is the spiral portion, and it is responsible for carrying the fluid and aiding noticeable difference between pitches.
Inside the cochlea is the scala vestibule, which is where fluid runs up to the apex of the cochlea, and the scala tympani runs down the cochlea to the round window.

Between the scala vestibule and scala tympani is the cochlea duct, which houses endolymph fluid. As the energy travels up the scala vestibule to the scala tympani, the energy must transfer through this cochlea duct.
Attached to the cochlea duct is a specialised structure called the organ of Corti. As the energy travels through the endolymph fluid, small nerves inside the organ of corti are stimulated which sends impulses to the brain via the cochlea nerve. These nerves are cells are called hair cells.

Covering the hair cells is a structure called the tectorial membrane. As vibrations are transferred, the tiny hairs on the hair cells press against the tectorial membrane which acts like a button press, triggering the cells to send the electrical impulse.

The hair cells are lined all the way up the cochlea. Different frequencies will excite different areas, meaning each hair cell is tuned to a certain frequency. The closer to the apex of the cochlea, the lower the frequency is needed to stimulate the hair cells, whereas higher frequencies produce vibrations closer to the base.

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We hear different frequencies at different volumes. So while the amplitude of a sound wave may be equal across a broad range of frequencies, we will perceive it at different volumes depending on the exact frequency through the design of our ears. This is reflected on the Fletcher Munson Graph.

When it comes to measuring the raw, scientific value of threshold of human hearing we use the unit dBSPL (decibel sound pressure level).
If you hear a pure tone, which is a sine wave, of a certain frequency at a certain dBSPL, you can track it down the line of the Fletcher Munson Graph to the 1kHz point, it will give you the value in phons. Take 100Hz at 30dBSPL. Following the line to the 1kHz mark gives 10 phons.

Measuring equipment does not record sound like this. That is why weighting networks are applied. Basically, the output loudness level will change to more closely match how we perceive volume rather than the raw values. Measurments with weighting will have weighting attached to the units, so for the A-weighting network, it is read in the unit dBA.

An important note when considering any loudness level is distance from the sound source. A particular sound will not emit at a certain sound level, but will be perceived as different sound levels depending on the distance. All measurement readings (throughout anything in real life but also on the Fletcher Munson Graph) will be at a set, specified distance from the source.

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Every sound has harmonics that give that sound a specific timbre (for example, your voice will have a consistent timbre because the harmonics are relatively unchanging).
When a sound source is perceived with a delay, either by the sound taking a longer path (usually by reflecting from a surface, called early and late reflections depending on if it reflects once or more than once) or by the sound being received from two microphones or ears at different distances, then some of those harmonics will be out of phase and destructively interfere. Other harmonics will also constructively interfere causing spikes in the perceived waveform.

Each of the peaks and valleys seen in a waveform are called comb filtering due to looking like a comb. This change in harmonics will change the timbre, meaning that comb filtering is a form of distortion (where distortion is change in waveform from the original sound source).

Typically the more of a delay or the more reflections, the higher the count of peaks and troughs.