http://maclantic.com/powerbookdefect/The%20Defect.html
There appears to be a bug in the powerbook's g4. Especially take a look at apple's lame workout:
http://docs.info.apple.com/article.html?artnum=302978
quote:
To avoid this, run only one audio application at a time. If it looks like you only have one audio application running but the sound is still skipping or stuttering, check for hidden login items or startup items.