Good information ProDigit.
I guess if I want to do this properly I'll need to a basic circuit like you've suggested.
I did however try it with a disposable speaker I got off a junk pile straight across 2 phase wires (in parallel) and while it worked quite loudly, as you said the power is way to much for it and it quickly blew. No doubt the 4000w and 72v my bike is running is a tad too much for the little 4" speaker I had on it
I'm not sure I can be bothered making the the circuit (basic as it is) for such a gimmick but the proof of concept is there if someone wants to pursue it.