[Coda]

What you're hearing here is essentially equivalent to a "remove vocals" filter.

The left speaker and the right speaker have different sounds going to them -- that's why it's stereo -- but due to damage to the wires or an improperly connected plug, those signals are getting merged together. Things that are the same in both ears (the vocals) interfere with each other and cancel out, while things that aren't exactly centered (stuff that's not directly in front of the microphone when it was recorded) don't have the same signal on both sides so they don't cancel out as much.

As an additional side effect, this means that you're only getting mono sound. If you have a song that has pretty strong stereo separation, you'll hear exactly the same thing in both ears when this happens instead of hearing the stereo effects that you would normally hear.

Why yes, I DO work for a recording studio. :P