Yep, you'll need to add your new SFX details into the SoundDialogue, SoundMusic or SoundEffects files depending on what it is. They can all be found in the GameInfo folder of your mod.

I just copy and paste earlier entries and then add my SFX details in.

You'll need to add your sound file to sound folder, obviously.

MP3s work too if you really wanted to use those for some reason. 

I never tried this, but there are 1 or 2  examples of using the ALT group, I think there are lots under muzzles (gun shot) in the SoundEffects.sounddata file.

Ya, just looked them up

effect
name "EFFECT_DEBRISDISAPPEAR"
fileName "Effect_DebrisDisappear.ogg"
type "Effect"
is3D TRUE
priority 5
threshold 10000.000000
minAttenuationDist 2000.000000
maxNumPlayingSimultaneously 2
isLooping FALSE
isResident FALSE
minRespawnTime 0.5
fadeInTime 0.5
fadeOutTime 0.5
alternateGroup "EFFECT_DEBRISDISAPPEAR"

few more

WEAPON_PHASESUPPORTMISSILELIGHT_MUZZLE
WEAPON_PSICAPITALLASERHEAVY_MUZZLE 

It looks to me like it may use any of the sounds with the name or with the name and ALT_1 or ALT_2 etc.

EDIT: Audio responses do not 'need' to be in stereo, in fact, I recommend mono ogg or mp3. Why waste resources on 2 channels for that stuff. Use different qualities of conversions to also save your resources. Explosions should be very HQ and voices can be lower quality, with everything else somewhere between. 

They may not all be loaded into RAM but there may be 20 or more loaded into RAM at any one time. Same goes for music, you may have up to 5 music tracks in your RAM at any time for seamless transitions.

The is3D TRUE is actually better for the game environment. The game knows where the sound is coming from on the screen and places it in the appropriate channels. How would it do that in stereo?

I have optimized every aspect of SOA2 and can play huge 10 player multi-star maps into the 20 hr range with not one drop of lag. Every spot adds up.

ah, so mp3 too is supported

I'm more of a WAV guy, personally.

erm, what?