Do not know what the problem is. You may want to try different cassets to see if it is isolated to the one you are playing. Adjust your treble and bass on your source player. Turn off the mic input. Turn on the line input.

I use Audiograbber and get very good results. You can get it free online. You can moderate the volume input and see it in the program. Also can set the quality of sound you want to have. Good luck.