Straightforward gaming feature, common to things like engine sounds--but also anything that "changes speed or intensity over time".
I'd like to propose llAudioSoundPitch(float pitch_multiplier)
This function would affect playback for the currently attached sound in the following way:
final_pitch = sound_asset_pitch x emitter_pitch x sound_instance_pitch x pitch_multiplier
Where final_pitch is the sound played by the viewer, sound_asset_pitch is the pitch sampled from the audio asset, emitter_pitch is any distortion up or down currently applied by the viewer due to motion, and sound_instance_pitch is the current multiplier already in place (if there is one).
Defaults are 1 for everything except the original asset.
Eg, subsequent calls of llAudioSoundPitch(0.5) at timed intervals will successively cut the pitch in half, altering the sound.
In this way, a single sound file can be made to serve many uses, decreasing the need for "speed up", "slow down", "idle", "full speed" sound assets.