I used to feed composite-video to my PC's graphics-card, and the audio to Line-in via a hacked SCART adapter. This was a pain with regard to switching inputs behind my PC, especially as the current video outputs audio in mono, and I had to bridge connectors with a paperclip to feed both L & R channels. This is pictured on the introduction.
As I've got a lot of bits from old stereos, TV's Videos, I had everything I needed to make a switching box / vision-mixer, so it was about time that I did.
(I use AV to mean Audio/Video)
Step 1: Parts
From an old stereo I removed (with much difficulty) an input selector switch bank. Pushing one of these switches releases any others that are engaged, so you only have one "on" at a time.
Out of a McDonnel Douglas P-LAN node, some wire.
From my kitchen, an empty mustard tin.
From a laser toner cartridge, some heavy black plastic sheet.
From I can't remember what, screws