Thanks, guys! Keep it coming.
Bobboau: Not sure how Doppler was used in retail, but I'm not sure where it would be logical to use it. Flyby sounds? Weapons fire (projectiles moving past you)?
Kolgena: No idea, sorry.
-Joshua-: Can you give an example or two of where distancebased sound morphing is used in HL2?
As for modifying the speed of sound and such, I wonder if this is where the EFX/EAX 2.0 sound environment presets that taylor added in the 3.6.12 sound code could be of use. My vague impression is that they currently don't work, but I really don't know. I'm not sure how much documentation there is on them, either.
More generally, It'd be good to look more into what OpenAL's EFX can do for us. I'll have to add that to the TODO list.
I would think the general rule for sound modifiers or any changes to the existing sound code would be
(1) does it sound good/better than before? and ideally also
(2) does it provide additional useful feedback to the player?
I suppose that immersion falls under both questions to some extent.
Also, Mjn, about sound stuttering when the frame rate drops, does that apply to all sounds? As in sound effects, voiceover, music? Can you check if it applies to sounds/music that are played via SEXP, e.g. play-sound-from-file?