Power Mac G5: Internal speaker doesn‘t mute if digital speakers are attached

When you have digital speakers connected to your Power Mac G5, your internal speaker will not mute--even if you select the mute checkbox in Sound preferences. But you can set the internal speaker's volume low (in Sound preferences) as a workaround.

"Digital speakers" means speakers that connect to the optical Digital Out audio port with Toslink cables, and use the S/PDIF (Sony/Philips Digital Interface) protocol.

To mute the internal speaker, disconnect the digital speakers. Or, if you like, use a different kind of speakers to avoid this issue.