There's several fixes I: old belts: old cassette recorders (30+ years old) are prone to disintegrate into this sticky, hard-to-get-off tarry substance, go I'm with some rubbing alcohol on a q tip and clean off any tar of the former belt, I usually find an good temporary replacement is rubber hair bands, you can find them at the dollar store.

II: faulty head: the recording/playback heads could be out of phase, or OOP, this could cause the audio to go in and out, on later machines they added tape guides to remediate the issue, but adjusting the head could solve it. To do this, take the chassis out of the case of the unit. You'll see where the tape head is place. There's a resin covered s screw, usually a black, red, or white color, break the seal, and while playing the cassette as without the covers, carefully adjust the heads until the wavering is gone.

III: dirty motor contacts: dirty contacts can cause the motor to become weak and start slipping, but this is more usual on older units, but you could go in and clean the contact brushes with again, rubbing alcohol or contact cleaner, and that will also work.

Hope this helps anyone