Head wear. Happens all the time, and on a cassette it's usually the left side
first, that's because it's the edge track.

On a decent cassette deck there used to be EQ and Dolby calibration pots,
enough of a control set to restore the sound if the head amp is quiet enough,
but I doubt it's possible on a boombox. On a good deck if you could actually
hear treble and it wasn't too hissy you could recalibrate it for the worn head.
You wouldn't attain original specs., but you could make it sound decent. In
your case you should probably replace the head. They are cheap if you know
where to get stuff. The problem is changing it.

If you go that route, when you align it run the output into a mono amp or a
switched to mono amp, I mean mixing both channels. This will allow you to do a
much more accurate azimuth alignment.

Since the problem happened all at once for you, it's likely that the head is
not completely roached, but the wear is making the oxide from the tape scrape
off into or over the gap.If you want to be cheap you might polish it or
something. . . . . .

JURB