Nigel makes an excellent suggestion. I have seen similar ideas based around a IR photoelectric type smoke detectors ( photo electric ones were easier to interface to since they didn't require the ionization chamber.) The smoke detector is used to control the main circuit, but instead of checking for smoke it powers up the main circuit, which in your case would be check for sound, then if no signal is present it goes back to sleep. The design also has built in battery check ( using a LED to load test the battery ) and alarm for low battery. The smoke detector itself can last nearly a year on a 9V battery, your circuit would reduce that, but atleast you get a complete controller for very little money. One such circuit is based around the MC145010 chip.
here is a link to the data sheet:
**broken link removed**