Just remember that cassette tapes are actually four tracks - two tracks either way. Therefore a mono recording on tape is still two of the tracks in normal use. When your tapes were brand new the sound on the two tracks would've been more or less exactly the same, if it was a mono recording. Over time there may have been some deterioration to your tape making a slight variation on/ between the two tracks. When you record from your cassette player record in stereo and then if you feel like it merge the two channels on your recording to mono. Be aware if you convert to an MP3 it will probably revert to two channels.