The Amazing Slowdowner from RoniMusic in Sweden is the program you need. It works on MP3s, WAVs, CD tracks. You can slow down whole tracks or just short bits, and the amount of slowing is infinitely variable, with the quality of the sound being good even when slowed down a lot. You can also change the pitch, if the recording isn't in tune with your instrument.

Find it here, and try out a demo version for free - it lets you use the first track of a CD. The password to turn it into a full function program costs (I think) £20.

The company is nothing to do with me - I'm a very satisfied customer trying to learn fiddle tunes.

Cheers
Ray