The left channel of my Terratec EWS 64 XL has seriously distorded audio when playing games or CD music. I have tried multiple speakers and headphones. MIDI playback is fine.

The audio on the left channel is too silent and has a lot of noise. It seems the card has some hardware issue. I have experience on soldering ICs and other SMT components so I should be able to repair it if I can find out what's wrong and get a replacement part but I have never done audio equipment diagnostics and repair so I'm not quite sure which components could be the source of the problem and what would be the best way to diagnose it. I have access to an oscilloscope so that might help with the diagnostics if I could get some advice on where to start.

Any ideas?