Whaddya know … I just reaized the F16 key behaved like I described earlier because of tapping it in a Safari textbox, e.g. like the one this comment is being composed in. The Cocoa text system seems to be one context for that style of beep, happening with certain unrecognized key combinations and silent with others. Maybe it's the ASCII BEL sound?

I like your idea of F16 doing a "visual mute" with iSight, though right now I'd rather it order pizza since I'm hungry.