Subwoofer auto on/off switch?

Hello fellow DIYers!

I have recently built a subwoofer for my home theater system, and I am looking for a way to create an on/off switch that detects incoming audio, and keeps the amp on for a minute or two after audio has stopped. I'm planning on using a PC power supply and a car amp(I have worked out the amperage draw and the PSU will handle it), so turning the whole contraption on can be easily done by connecting two pins on the motherboard connector. I was thinking I could use a relay connected to the audio in that would turn on the amp if the voltage was above 1V or something, but that wouldn't work well because I think the audio would be AC and would turn the amp on and off with the waveform. Also, the amp would shut off immediately once audio ceased. Any suggestions would be appreciated!

Phil