General descriptions:
* Rustling
* Crinkling
* Scrunching
* Wriggling
* Squelching (if it's very wet paper)
More specific descriptions:
* Crackling (for crisp, dry paper)
* Shuffling (for thicker paper being crumpled)
* Thumping (for a heavier crumple)
* Whispering (for a delicate crumple)
You could also use onomatopoeia to describe the sound:
* Crunch
* Scrunch
* Crackle
* Squelch
Ultimately, the best word to describe the sound depends on the specific type of paper, how it's being crumpled, and the desired effect.