Would this happen all of a sudden though? I had been using the deck immediately before that for about 1hr.
Not questioning your help and input in anyway just trying to narrow it down with as much info as I can.
First, try the easiest solution -
How do other tapes sound? How does the newly muffled tape sound in other decks?
If all tapes sound bad, just give the heads, capstan, and pinch roller an honest cleaning. SOMEtimes yeah, it is as simple as something came along and just gummed up the head, something nasty on the tape ribbon.
Second - mistracking is possible but if the head were to work it's way out of alignment, that is usually a gradual process.make sure the tape seats properly. Make sure the non-adjusting screw of the playback head is secure. It is the one on the right side of the head. I recently picked up a stereo with a built-in cassette and mine was loose. Snugged it, problem gone. Yeah somehow it was pretty loose.
The screw on the left is to adjust azimuth. Maybe tweak it a bit (if you are comfortable doing so) until you get the best sounding treble. Do not remove it or you may have a time finding it. The safest way to adjust it would be to give it a couple turns in and out and you will hear the treble coming back. Getting it perfect is difficult, as even fractions of a turn can make a difference. You should also hold your face in a special expression while doing this for best results.
BTW, the door on most cassette players is removable (at least the cover) which will give access to those screws while the tape is playing. Try to use a non-magnetized metal screwdriver though. Those plastic calibration screwdrivers will just break cause those screws are not as easy to turn as one might imagine.
