Audio sounds oddly distorted
It's mono, not stereo like one would expect, and it sounds like exacerbated compression artifacts to me, like in the difference signal between the stereo channels of a lossy compressed audio, but I'm not sure if that explains everything.

Given that he uses an external mixer for recording, there may be a simple explanation: He has connected a stereo cable to a balanced input of the mixer. The balanced input will typically take the left channel as "hot" and right channel as "cold", and will effectively see the mono difference signal: hot minus cold. He should use a stereo to 2x mono splitter cable, or use a stereo input in the mixer if available.