This generally fixes it:
Resetting the SMU on a G5 iMac:
and on resetting the PMU on a Power Mac G5, G4 and G3 :
Did this happen after you used headphones or a set of external speakers that required you to use the audio-out jack on the PowerBook?
If so, see this thread:
It's about a different Mac model but the PowerBook shares the same audio port technology so the suggested fixes and my bit about the cause and future prevention apply to your computer as well.