Require assistance

Windows 7, Audacity 2.1.1,.exe installer. Two questions: (1) I recently obtained an audio cassette player that converts the music or dialogue over to the MP3 format. As this player does not have an auto-reverse feature, I ended up with two MP3 files. When I imported the both files onto audacity and then played what I had transferred, I received two identical versions playing at the same time.How do I go about combining both files into one?;(2) I have a voice cassette that I had loaned to someone many years ago, and they inadvertently managed to get music over the dialogue.Is there any way to eliminate the music? Thanks for your help!

Audacity has an option to “split stereo to mono” … Audacity Manual
If the tracks are truly identical , (a/k/a dual-mono) , you can split the “stereo” (dual-mono) pair , than delete one of the two tracks created.

If they are not truly identical , actual stereo, not dual-mono , then the option to go for is “Stereo track to mono”, which is in the “Tracks” menu … Audacity Manual

Generally no.