How to lower or mute the Mac bong?

Random question, but whenever I start my new iMac it makes the usual bong but it's loud, it doesn't even come out if the system I have hooked up to it.
I can live with it, but our neighbors are anal and not too keen on us making noise.
Is there a way to mute it or even just quieten it so it isn't as loud?

Cheers