How can you fix a tape player when it plays really softly?

If the tape is playing at the correct speed, but the sound is muffled - it's likely the heads need cleaning. Remove the cassette - press the play button, and clean the exposed head gentlyfor a few seconds, using a cotton bud dampened with some methylated spirit. You'll see a dark brown or black smudge on the cotton. Throw the cotton bud away, and repeat with a clean one - Repeat until the cotton bud comes away from the head clean.

I've used this method for thirty-odd years - and find it works better than using a 'head-cleaning cassette'