I don't see how it can happen. Guess your the first.
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The explanation for the rap artists is easy: there isn't any music there regardless of what you play it on. The others are harder to explain. It could be summing the left and right channels resulting in the lead vocal doubling in volume while the other elements remained about the same.
On the car CDP, it could be a 180 degree phase inversion of one channel resulting in the practically mono lead vocal track cancelling out while the more left to right panned instrumental tracks wouldn't.
Most likely there was a phase reversal of the speaker leads hooked to the monitor amp used with the computer or within the monitor amp itself, not on the audio card in the computer. To highlite vocals or instrumentals, you need to try a phase reversal game on playback. It might be as simple as reversing the plus and minus on one set of speaker leads, but better yet would be one of the old McIntosh preamps that gives you all kinds of phase-relationships: stereo reverse, right only, left only, and various combinations, either fed to one or two channels.