Hmm - sounds like an interrupt-related issue. Moving your finger on the track-pad (or, presumably, typing on the keyboard) generates frequent PS2 interrupts, which also allow the audio sub-system to process as needed. Without this, audio events aren't processed in a timely manner and so sound is choppy and of poor quality.
Are (some) interrupts disabled on sleep, and the VoodooHDA driver is failing to re-enable them (correctly) on resume?