If you run 'sudo kmodstat' do you see any names with Audio in them loaded? On my DP G4/500, I see three:
com.apple.iokit.IOAudioFamily
com.apple.driver.AppleDBDMAAudio
com.apple.driver.AppleScreamerAudio
If not, see if you can do a kmodload of the right extension(s) from /System/Library/Extensions/AppleOnboardAudio.kext/Contents/PlugIns (not sure of the exact syntax for kmodload, but just running it without any args gives a usage).