Does the music sound distorted from a cassette? If so, I suspect the tape player is on its last legs. If the radio sounds bad, then that could be the stereo, the aerial, earthing, the speaker wiring or the speakers (and maybe some other things too).
Register Member 829
1980 Mk1 GLi
1980 (1976) Mk1 Junior Cup Replica
1982 Mk1 GTI (pending project)
1987 MK2 GTX