Slowing down the playback of an audio file without changing its pitch?

Timestretching is quite hard. The more you slow down or speed up the sound the more artifacts you get. If you want to know what they sound like listen to "The Rockafeller Skank" by Fat Boy Slim. There are a lot of ways to do it that all have their own strengths and weaknesses. The math can get really complex. That's why there are so many proprietary algorithms.

This page explains things a bit clearer than I can and links to the Dirac library. http://www.dspdimension.com/admin/time-pitch-overview/

I found this link for java code to do pitch shifting/timestretching http://www.adetorres.com/keychanger/KeyChangerReadme.html

I use soundstretch to speed up podcasts which is works quite well, haven't tried it on music though.