I'm having another problem listening to tapes on my WM-10II.  When I start at the beginning of the tape everything sounds fine, but after a few songs in, everything starts to sound slow and warbly, even on a fresh battery.  The strange thing is, when I flip over the tape, the same thing happens.  The first couple songs are okay but after that they're bad.  I'm thinking it could be the tapes which I picked up a flea market, they may have been magnetized.  Unfortunately I don't have any known good tapes to test right now and see if that is the case.

Is it also possible that there is another problem with my Walkman?  The belt I am using is very slightly thicker than the original, but I was hoping it would work, so I hope that's not contributing to the problem.

"Slow and warbly" and wow and flutter is usually mechanical. It can be the tape's fault if it is poor quality and binds up- to test this just try turning the reel with a pen and see how much resistance there is.

However, I think it is the walkman. Having just fixed a WM-F20 (same mechanism) it really does need a perfectly sized belt to work properly. It seems the disc motor has very low torque and makes up for it with speed. Unfortunately I don't think it is the most reliable Sony but it is a marvel of engineering- when it works.

(Other stuff to check are pinch rollers and lubrication)

Hmm, I see, thank you.  I would think it's the Walkman if it didn't sound fine on the first few songs... I can even rewind back to the beginning and it will play fine.  So I'm really hoping it's the tapes themselves, because they were cheap.

I will check to see if anything else is binding.  If this problem is caused by the belt, luckily I'm getting some proper ones from DocP.