FYI, from apple:
http://docs.info.apple.com/article.html?artnum=302978
When you are running two or more audio applications on your PowerBook G4 (15-inch Double-Layer SD) or PowerBook G4 (17-inch Double-Layer SD), the audio may intermittently stutter, echo, or skip backwards. This can happen even if one audio application isn't playing or you have the sound muted in the application (such as when playing a game).
This can involve any application that uses audio, such as QuickTime, iTunes, iDVD, and many third-party applications like games and audio applications.
To avoid this, update your computer to Mac OS X 10.4.5.