Okay so i just changed the belt and felt pads on my M9830K, the only thing is that the sound from the cassette is not as loud ( in terms of highs ) as the radio. The radio sounds a lot more crisp than the tape. Tape playing sounds a bit more muffled. I tried several tapes. Is this just how the box sounds or could there be another fix that needs to be done.?

Try cleaning the tape head first.  If it still has poor sound then you might want to try adjusting the tape head alignment with a small screwdriver.