If it is reading the discs ok, and spinning /playing, then the laser can probably be ruled out as the cause. However, faults like these are hard to fix without knowledge and the schematic diagram, unfortunately.
Is the source selector a manual (clunky!) switch, or electronic , i.e. do you press a button to select between cd/rad/cassette?
Another tech I know once had a similar problem to yours with a portbale with an electronic selector. It turned out to be a chip in the unit which had got corrupted somehow, and was not selecting the sources properly.