Sounds like good advice from Sparky.
But if you're still stuck, here's a bit more info.
It's most likely oxide on the audio head or bad tape path due to a worn mechanism, is the cause of the problem.
Audio/control heads often get dirty but very seldom fail themselves.
Make sure to disconnect from the mains supply.
Viewed from above & front, the audio/control head is the vertical static tape head to the right of the spinning video head cylinder and is situated behind the rubber pinch roller, next to the capstan.
You will see two small silver strips on the face of it. The upper one is the mono audio head and the lower is the control head.
You can clean both with a cotton bud and some head cleaning fluid. Oxide ('dirt') often shows up as a brownish mark. A store bought video tape head cleaner is unlikely to remove it, as it cannot provide sufficient pressure, so manual cleaning is preferred.
If the head is clean then the likely cause of poor sound is bad tape path, which may have damaged the tape itself. Check for damage to the edge of the tape.
You might also check/clean the erase head (usually black) on the left side of the cylinder and ensure the connections to it are secure.
If simply cleaning the head doesn't cure it, replacing this budget mono video with a new up to date model makes more economical sense than taking it for repair.
Note: Although there are some adjusting screws to adjust it's tilt/azimuth, I 'strongly' recommend against adjusting' them as the head position is very critical. Even an engineer would only adjust them with reference against a special alignment tape. Any adjustment to the screws would likely be very, very minor, say quarter turn tops) but try not to be tempted, they're usually best left alone, except as a last resort or you'll end up in a pickle.
(PS: You get twice the error on your own recordings as the tape passes over twice, once to lay down the recorded track and a second time to play it back. On pre-recorded tapes, you get some sound as the recorded track was laid down ok but the playback only is faulty.)
Cobweb, May 2006