headphones don't solve the problem. Generally speaking, all the audio on the computer comes out of my monitor speakers, routed through a MOTU soundcard. Apparently, only the startup sound doesn't come through that pathway, instead it blasts at top volume through the speaker in the computer itself, obnoxiously distorted. None of the normal alert sounds do this...they respond to the volume changes in the "sound" preferences. However as I think I said in my original post, the "overall" volume control, the one that appears on the toolbar at the top right of the screen next to the date and time and shows a "drop-down" fader when it's clicked on, is unresponsive to anything I've tried.

Under the heading "Sound Effects" I've set alerts and sound effects to play through the internal speakers. "input" and "output" are both set to the MOTU PCI card that plays audio through my studio monitors. The "output volume" fader at the bottom of the window is greyed-out and doesn't respond to attempts to move it, neither does the fader in the menu bar should I choose to show that...