I have been using Audacity for a number of years on a laptop and desktop computer. It is a free open source program for working with audio files. I was curious about this discussion and wondered how Audacity might be useful. By adjusting the audio setup, I was successfully able to record a guitar backing track on Audacity in real time. ( I used the “Back Up And Push” backing track @ 110 bpm ) I then selected the file and used the built in ‘tempo change’ plug-in that comes with Audacity. I changed it to 85 bpm with out changing the pitch. The file can be changed as a percentage or by bpm.
Next, I recorded the “Back Up And Push” backing track @ 85bpm onto Audacity. Perfectly aligning the two files is beyond my capabilities, but it was easy to hear that the two files were at the same tempo. I did not save the files. Audacity can save an audio file in just about every format, MP3, WAV, Flac etc.
Audacity worked well and was easy for me. As a caution, Audacity is very powerful program and some things take time to learn. But the basics are pretty easy.
Happy Fiddlin!