I know it is giving me lots of problems also. As i am trying to write a mod player pitch is way to limitied.

I decided to see if i could change the frequency of the buffer. This did not seem to do anything at first, but...

Deleting the buffer and recreating it does it.

Take a look at the modplayer source where i delete and recreate the buffer to see what i mean.

You can use the frequency on your buffer to make it sound higher or lower.

Be aware to stop the source before deleting the buffer or else a memory leak will occur.

Personaly i thing you should use multiple samples for your engine sound. Just changing the frequency (or pitch) is not enought to make it sound realistic. Maybe a sample for every gear?