If... ground of the amp can be shared with Arduino ground, then you only need three resistors to measure AC output voltage of the amp.
One from amplifier speaker output (hot) to an analogue input, one from analogue input to Arduino ground (the normal voltage divider), and one from analogue input to Arduino VCC (5volt). The resistor to VCC pulls the pin up to about 2.5volt, so AC measurements are possible.
Try 100k between amp and analogue pin, 4k7 from pin to VCC, and 5k (2*10k parallel) from pin to ground.
That should do ~200volt peak/peak.
Post#3 in this thread should give you ideas how to get peak/peak value.
From there you can calculate the rest (if you use a resistive load).
Leo..