[Five]

Notice how the phase of the right channel is reversed! A-ha!! I couldn't find anything similar in the left channel (not to say it isn't there). I suspect that your right channel is out of phase and some of it has been blended (and the part blended into the left is IN PHASE ) with the left channel. Maybe the part that's been blended is in phase or out of phase, I'm not sure. I'd also guess that some of the left channel is somehow blended with the right, for sure it is during the "left channel mono" parts. What scares the hell out of me is the randomness of it all. Mixing channels and inverting phase suggests some kind of "super-surround!!!" kind of thing going on. But then that wouldn't explain the randomness.

Just in case, check Start > Settings > Control Panel > Sounds & Multimedia > Audio > Sound Playback > Advanced > Speaker Setup and tinker around with the settings in there and see if anything changes. This probably won't work and you've probably tried this, but I thought I'd mention it.