Since this is my specialist area, I'd love to help. But I am currently sitting on a balcony overlooking the Ionian Sea in Greece, and only wearing a towel, so I'll keep it brief:
* The duration of a WAV playback is completely deterministic unless the audio sub-system is rubbish. What may be a problem is the accuracy of the timer used to determine when playback has started finished. The start time may be later than simple maths predicts.
* Not all hardware will allow tweaking the playback rate outside the usual "windows" - 8,000, 12,000, 22,050, 44,100. Consequently, the waveform needs to be modified, or a suitable range of samples at different frequencies used.
* For effective music production with virtual synthesizers you need to able to determine the attack, hold and decay sections of the waveform and probably loop inside the area between the attack and decay.
* Ideally, any mixing is done in high precision in a system of ring buffers.
Personally, I don't think it's AGK's job to provide this in Tier 1. It's all possible in T2 or Pascal, using platform-specific code.
-- Jim DO IT FASTER, EASIER AND BETTER WITH AppGameKit FOR PASCAL