Ok, so I initially had a CDT file, converted that to WAV using TAPE2WAV which gives me that
1-channel 8-bit WAV at 44100Hz @ralferoo mentions and then I made it mp3 using Lame/Audacity.
When I created the mp3, the software asks me which version to use, there's more compatible v1 & more flexible v2, in this instance I went on compatibility, it also has information concerning it's title and any other details about the file itself, along with a pop down box for Genre. It probably doesn't matter what's in that box, but I selected Other, just in case it wanted to play around with the tone.
I also had to do some editing. The first mp3 I created was loading fine up until the end of the 5th block of the program, when I got a Read Error A. I didn't have this problem when the file was in WAV format, but what happens when TAPE2WAV does it's job, the last block of the program ends abruptly, I simply copied some of that silence, lined up my cursor to the end of the last block (which displays itself as a solid block), and pasted it there to get the file loading on my 6128.
I also mention a Read Error B, which results when Data hasn't been correctly received to the computer, unfortunately I don't know if it's the result of some mp3 degeneration which the human ear could not pick up, or if it was because I was moving a monitor (with interference on it) towards the reader. Once I moved the monitor back to where it was, the file was loading fine again though. I just didn't want to find out that I could make all these mp3s and then they become unreadable. I since found out the interference from the monitor was coming from my disk drives.