For what you are doing, the M-Audio Audiophile 2496 may be just what you
need. It has two analog in, two analog out, one stereo SPDIF in, and
one stereo SPDIF out (plus MIDI in/out). I think you get it for under
$100, and it has great Linux support (including the envy24control
utility), plus it has 36-bit DSP, hardware mixing and monitoring. This
card does NOT have microphone inputs, you will need a mike pre-amp or
mixer w/pre-amps for that (M-Audio makes a nice little mike pre-amp also).