No useful sound output, only the screaching, from internal speakers or from sound-out jack in this heavily-upgraded '98 WS (Sonnet G4/500MHz, 512MB, 20GB drive) that has worked beautifully in current state for almost two years (OS 9.2.2 and 10.2.8 on separate partitions).

Considered it might be the Power/Sound board although I am having no power or charging problems now. This board was replaced once about three years ago for failure to accept a charge.

I can make sound adjustments in Sound pref panel in OS 10 but it only serves to increase the volume of screaching when any sound plays. Curiously, when booted in OS 9 and accessing Sound control panel, "Built-in Audio Controller" is greyed-out and any attempt to use the volume slider is met with the slider snapping back leftward and an error dialog with a message something like "Cannot access audio controller as it is in use by another application."

Now I am not sure what to think - ?hardware...?software...

Any thoughts?

Software related sounds like. Try booting with extensions off.