I am using a new USB Phono Plus Computer interface to create digital files from Line input from a dat tape. I stop at the end of a track, export from Audacity2.5 to a hard drive. But all the files contain ALL the music from each file. So I have a cacaphony. What am I doing incorrectly here.?
Audacity 2.5 does not exist. Please download the latest official release from here.
Please have a look a this tutorial, especially section 5 on splitting a recording into separate tracks.
It sounds like you have multiple tracks in your project, and when you export, Audacity is mixing them together (as it is designed to do).