I would love to hear what you guys have to say on this subject, what tricks you know of. Especially you long time audio engineers.
I’ve been an audio engineer for over 20 years. I have mixed, mastered, and engineered many projects. Most of which were music, but I have done some sound design including weapon sound design.
What I hope to do here is impart some of my knowledge to you, learn new tricks and ideas, and get some of the principals in use in the game. I am sure some of you already know this stuff or just haven’t thought of how it would apply to UT. You might also note that some of this has been used in UT games of the past.

Visual clarity is something that everyone wants in this game. There should also be aural clarity. It’s not a glamorous subject but it is important whether you know it or not. This aural clarity will help players on a more subconscious level that visual clarity does as visual clarity is much easier to notice and quantify.

We have a few tools and tricks at our disposal to make the audio clear and concise. We want to be able to know exactly where our opponents are. When, where and what kind of items are nearby. We want to know if a sound is important, ambient, music, or a notification. If a sound is important, it must take precedence over an unimportant sound.

Here are some ways to achieve this clarity.
Here is a trick used a lot in mixing music to help a dense mix sound clear and help individual instruments sound defined and not buried. Besides stereo separation, we also have frequency separation. The idea is to use a 1-band parametric EQ to carve frequency spaces for sounds depending on how important the sound is. But not so much that it sounds unnatural, just enough for some added Clarity . For example center frequency approximately 1.5KHz (frequency range that the human ear is very sensitive while not being too high of a frequency that it would sound unnatural and annoying). Important sounds like footsteps and weapons would have a wide bandwidth boost at the center frequency of perhaps 4dB of boost. The amount of boost would need to be tweaked once all the sounds are put together, same with the center frequency. But it will for sure need to be a wide boost. For the unimportant sounds, like music and ambience, there will be a fairly narrow cut at the center frequency (the SAME center frequency used for the important sounds). This frequency separation combined with stereo separation should help make it so players can actually turn up ambience and music more than usual while still being able to locate enemies and pickups.

Engine & game settings:
The frequency separation could actually be a setting in the game instead of done in mastering, with a slider ranging from about 800Hz to 4KHz (or locked at ~1.5KHz) and a “Separation” amount (i.e. boost / cut). It's the same principal as I mentioned in Mastering. The EQ should have a wide boost for important sounds like footsteps and weapons, and a fairly narrow cut for ambient sounds and music. Stereo separation controls for music could also be a setting. It could be a slider that can make the music full stereo / 100%width (default) all the way to full mono / 0%width (as I mention under “music” heading). This will be a compromise for more serious players that like the music but also want to be able to hear the important game sounds around them instead of just muting the music as most players did in the past. Obviously there should be volume sliders for different sound groups as well.

Mixed mono, very low dynamic range – pretty much clipped (i.e. LOUD). This is how past UT’s were and for good reason. The dynamic range for all the individual weapons should be mastered at the same time by the same mastering engineer so that all weapons are processed in such a way that they all have similar apparent loudness. This will help the sounds remain all on the same level of quality (sounds coming from multiple people and sources). Players need to be able to hear these sounds very well. An audio purist would say more dynamic range is better, and in most cases that is true but not for a UT game weapon. The weapons from enemies will appear localized to the enemies’ locations on the stereo field and on the 3D map. For your own weapon it will be either stereo or mono directly in front of you. This will help differentiate between your own weapon and your enemy. We can further accentuate this differentiation by giving different EQ curves to your weapons and enemy weapons and perhaps give 1st person weapon sounds higher dynamic range so they are not quite as prominent as the enemies’ weapons. But not so much that our own weapons would sound weak, they must always sound powerful.

Need to be attention grabbing, they need to be easy to hear, yet easy to ignore. They should have a mono component with a stereo effect to catch your attention.

Footsteps, grunts, & pain sounds:
These should be mixed mono, and have very little bass frequencies so there isn’t an overabundance of footstep bass “thump-thump-thump” all the time. Plus bass frequencies are either really loud sources, or are very close to your ear. Usually an enemy is far enough away that his footsteps would not have those bass frequencies (aka Proximity Effect), and footsteps are definitely not considered a loud source.

Pickups /respawners
Mixed mono. Pickups should have unique respawn sounds, and in the case of valuable pickups should also have an ambient sound once they have spawned.

Mixed stereo, yet can be made narrow stereo field or even mono. This would be awesome to make as a game setting along with volume (a “Music Width” slider or knob). As long as there is a way with the audio engine to narrow the stereo field in this way by summing the music channels.

A stinger is a sudden loud wide stereo sound, usually used in times where the game intensity is high. The purpose of this sound is to “reward” the player while helping the adrenaline of the player stay high.

Higher dynamic range, mono and stereo sounds. Things like machinery, light buzzing, birds, frog, fire, etc should be mixed mono. Whereas things like a machine room, many frogs, many birds, wind, cave sounds, etc should be mixed stereo.

Currently there is a low pitch wOOw sound when you hover over selections on the UI. This is exactly the opposite of the sound we would actually want. While it is a nice sound, it isn’t used in the right way. For hover sounds, we want a quick and crisp sound (like UT99’s tick sound on the interface). It needs a sound that is instantaneous and happens as fast as the UI animations are. Right now the hover sound is very sluggish making the interface “feel” lagged when it is in fact very quick. The current sound would be good for screen changes, or something that has an animated effect or is in some way a little slower.