cocoadev said:
...is it possible? What is the voltage that drives a PA soundsystem? I presume it fluctuates +-5v? So if I took a feed from the speaker I could light an LED if I chose the right resistor.
The voltage is going to be considerably higher than that!.
The formula for power is W=V*V/R.
So V = Square Root(W*R).
Assuming your system is 600W RMS into 4 ohms (which if it doesn't actually state RMS is probably a great deal less!) it works out at 49V RMS.
Which is about +/- 59V using your original method.