Probably not the startup bong on the 128K. It should always make that bong unless something is plugged into the headphone jack. If the brightness knob works, then he still has a problem with the sound somewhere. I actually forgot it made no bong, so more than likely it is a bigger problem than simply turning up the brightness.
I've never tested it, but I suppose it's possible the sound control panel settings can be saved in the PRAM. However, that also assumes that it has a working backup battery since those settings were last adjusted. And it assumes that any startup disk he's using also has the same settings (which would override saved settings). But it would be easy to check, just take out the Backup battery, and start it up. Somehow I think the 128K does not save sound settings though as I've never heard one bong softly.