iPhone: What is the best approach to changing the sound of a rolling ball based on speed?

Question

I need to play a sound of a ball rolling and change the sound based on the ball's speed. I was using AVFoundation and just playing different samples but that didn't sound natural enough. I tried OpenAL and something like:

alSourcef(sourceID, AL_PITCH, thePitch);

and passing in thePitch, but didn't like the results.

Does anyone have any suggestions or examples / tutorials?

Thanks.