not sure if it can do it, but try Audacity, a decent free audio editor. I don't use it, so I'm not sure, but it might have the feature to play a file backwards.
Audacity does have this option under the effects menu. Just select the entire song (if you want to play the entire song backwards) or a specific part and click reverse under the effects menu. Audacity is available for download from afterdawn on this page: http://www.afterdawn.com/software/audio_software/audio_tools/audacity.cfm
