Drift - Coax will also carry an analog signal....just think about your old cable signal before digital cable. This being the case, Glacier should still be able to achieve what he's looking for.
Glacier - There are adapers out there that convert from coaxial to rca (coax). Radio Shack should have them. If not, you can get them on the net (
Parts Express for example). That is the only way I know to join coax and rca. The problem is that it's only going to carry one of your two chanels of material, unless you have two coax runs.
An alternative would be to use an modulator to scoop up both channels and insert them onto the coax line.
In the end, you might just want to find a sound card w/ a digital coaxial output, as Drift suggested. Then, it's just a simple conversion using the above adapter and no mini/rca conversion. If it's an issue, I'm sure you can find one that's affordable.
Good Luck