[Hampshire] [OT] Mixing PC sound output

Hi all - seasons greetings

Here's the problem: I usually have several machines on at the same time and I'd like to hear their audio output (even if it's just a bong, or the you've got mail sound). I don't want to hook up speakers to each machine (one XP, one Ubuntu and one Mac) so at present I have them going into a hifi amp via 2 separate channels on a "8 channel" stereo mixer. Trouble is, the other 4 channels on this supposedly 8 channel beast (it is very very old) aren't available at line level - only mic level so it would sound terrible if I jacked one of the PCs into effectively two mono channels.

Are you with me so far?

Are there any gadgets out there that could let me sort this? Maplin have some but they're the same configuration as my current i.e. 2 stereo and 4 mono with each mono mic level only.

Cheers - hoping this isn't complete jibberish.

Rob