I think what you're looking for is "Voice Activated recording", much like how a security camera with motion sensor works in that it starts recording when the input to the sound card is over a certain threshold, and pauses when things are silent.
There are also auto-started programs that can be set to start and stop recording at certain times, like a DVR.
There are a few out there, this is what I could find:
Triggered:
-ScanRec. The original website is down, but it's available from Softpedia, Software Informer, etc., just do a search for "Scanner Recorder" or "ScanRec", you'll find it. Only records to WAV format, but does have a buffer so it starts the audio recording from just
before the trigger, so you don't get clipping. Freeware
-VoxRecorder
http://antoninoporci...xoom.it/VoxRecorder/Same as above, don't know if it buffers though. Freeware
-ReCall
http://sagebrush.com/recall.htmAs requested, this runs in the background, listening for audio. Not free; $14.95 or ReCall-PRO for $29.95
Timer-based:
-AutoRec
http://www.scott-inc.com/html/AR.htmFree.
Both:
-LoopRecorder
http://www.looprecorder.de/Can run in the background.
Free, with a Pro version for way too much money, but apparently NASA used it.
-Audacity
http://audacity.sourceforge.netHas both
Timed Recording and
Sound-activated Recording features. Free and Open Source.