Almost always a wiring problem.

If the file itself been recorded on one channel only then this points to an input problem. Look at the file using a decent audio editor to check.

Then double check that all line level connections to your soundcard are made using stereo connections, and that they are all plugged in correctly.

To verify playback - do mp3 files play back in stereo? and is it the same using headphones?

Have you checked all the volume settings (both playback and record)?