It would be very useful if there was a gain knob for every notification sound individually.

Another feature I would love is a sustain toggle on sounds individually, so that when trigger frequency is shorter than the sample, no audio would overlap.

Neither of those are possible. Addons can only ask WoW to play a sound on one of the existing sound channels in the game (which is already configurable in TMW's general options). Sound level of individual sounds cannot be controlled, nor can addons know how long a sound file is. I'd recommend setting the trigger frequency to a value longer than the sound file you're playing.

Ah.. that's a shame. Such a limited API, and I should have known. Thanks anyways.