In Debian, one of the things that didn't work out-of-the-box on the iBook G3 was sound. Forensic research reveals that it's all due to one G5 somewhere in Germany many years ago that went on the fritz when loading the PPC sound module "snd-powermac" during testing (At least this is what I remember from scanning the many forum threads and mailing lists on the subject. But I may have been stoned.). So it wasn't activated by default. This isn't a problem for people with newer PPC machines which use the "snd-aoa" module (Apparently. Again, stoned.), but for people with older Power Macs and 'Books that require the snd-powermac module, you can see how this will cause difficulties.
Luckily it's easy to load the module yourself. First open a terminal emulator and enter:
Now test the sound on your Mac. An easy way is to backspace at the terminal prompt--you should hear beeps.
If you hear nothing, then I can't help you. But if you hear sound and you want to keep it functioning on reboot, simply add the line
to /etc/modules and you're all set.
Also, if you're working from a minimal install, be sure you've installed the packages alsa-base and alsa-utils. Alsa is the sound driver, and it's of some importance to the above.