I love the idea of ringing a bell for a sale (when you getting higher rates you could ping less often with a different pitch). I think sounds are a great side channel for event notifications.
IIRC, in one of Wil Shipley's presentations, I think he mentioned that they did this at Omni, and while the constant ringing of cash-register sound effects was nice at first, it got old fast.
I think that definitely counts as a "nice problem to have" :-)
Oh, I definitely agree, but after a while it might make sense to only play a sound every n sales. :)