The power that is transmitted is low, probably 12v.
First: Power is measured in Watts. Electric potential in volts, and current in Amperes. But we call them voltage and amperage for short :-)
Have you measured it? 12V over a 8 Ohm speaker would be 18 watts of power; that's a pretty loud alarm (car stereo cranked up level.)
So, you have a speaker, and you want to have a USB device that detects whether the speaker is making sound louder than X, or not. I can think of a number of ways of doing this:
1) You can use an inductor next to the speaker, that energizes and generates voltage when the speaker is on. Then use a rectifier, a capacitor, and the ADC input on an Arduino to read this voltage.
2) You can use a microphone to read the output of the speaker. (OK, corny, but it would work :-)
3) You can couple a rectifier bridge to the speaker. Use small signal diodes with low forward voltage drop. Couple a capacitor and resistor (to bleed the cap) across the rectifier output. Then couple the rectifier output to an ADC input on the Arduino. I'd use a 10 uF capacitor and 10k resistor.