I am looking for code for slowing down music without altering pitch.

If anyone is interested, here are some programs that are designed for
doing this for music transcription. A couple examples here with free
trials:

Amazing Slow Downer: http://www.ronimusic.com/
Transcribe!: http://www.seventhstring.com/
Slow Gold: http://www.worldwidewoodshed.com/products.htm

I'm looking to do something similar (for a non-commercial app) but
with different features.

I think that some softer (Transcribe! above) does this via Fourier
transform. Is that the best way to achieve this without reentrant
glitches?