butchb, you are fabulous!!!
I just tried your suggestion, and it works day after day:
"hi i have a Imac mid 2010 & 2011 models so far mine doesn't do that GONG sound by following this easy steps:
1st unplug your speaker then go to System Preferences, Sound, Check the "MUTE" box then plug your speaker back. You will notice that the "MUTE" will automatically uncheck itself after that just restart your mac & there you go no more annoying GONG sound."
Whenever I reset PRAM, I will simply redo what you recommended. Takes seconds.
For all of you out there, I have an iMac purchased in December 2011, and running Mac OS X 10.7.5.
Use butchb's suggestion. You will be pleased!