For Googlers wondering how to do this using Audacity or similar:
Start up Audacity and set your recording preferences. Edit > Preferences Quality. You want at least stereo and 41kHz, which is CD-quality. Higher won't hurt, but it will take up more space and since you're recording from a (generally) poor-quality source, it's probably not necessary.
Set your source to Stereo Mix. Play a sound file to make sure the levels come up. If you don't see red moving bars, it won't record anything.
Once you're sure you'll be recording, clear that track or start a new one. Exit any programs that will cause noise - IM programs, etc. You don't want random beeps in your recording.
Press Record and then start playing the file. You can edit out the silence in the beginning later.
Audacity doesn't export to MP3 by default, so either choose a different format or get the LAME dll here: http://lame.sourceforge.net/index.php
Audacity will ask you once where to find it (unless you move it of course) so save it in your preferred location and then point Audacity at it.
Disclaimer: As of 10 June 08 this was written from work, so I may have skipped or bungled steps.
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