Interestingly, many years ago I had this start happening with my car stereo. The vocals were mostly missing but the other instruments were there. When it got to the instumental solo, the main instrument was missing. This happened on 90% of songs.
Turned out I had accidentally caused wires to come loose in the trunk, leaving the speakers wired in series, which caused the stereo channel cancelation effect.
It's a common problem with headphones when the wire becomes damaged, allowing two audio channels to become electrically connected. One could even simulate this effect by delibrately not inserting an audio jack all the way in on some devices.
Turned out I had accidentally caused wires to come loose in the trunk, leaving the speakers wired in series, which caused the stereo channel cancelation effect.