no sound except startup chime (G4 PowerBook + OS 10.5)

Good morning.

"All of a sudden" I am unable to get any sound except the startup chime. All internet sound sources (like Amazon clips and YouTube) are silent.

The sound volume slider on the menu bar is "frozen" (ie unresponsive).

The sound preferences are set to builtin audio for both input and output.

I am unaware of any changes I've made recently.

Any ideas?

Thanks for your time.