Is there an app...

[acosmichippo]

...that will play back iPod music at varying speeds?

[Napoleon_PhoneApart]

I don't think so. The iPhone as it stands only has native variable speed control for audiobooks.

[psylichon]

Apple doesn't allow that kind of access to the iPod library. Probably the closest you'll get on the iPhone is something like this:

http://www.ronimusic.com/asd_iphone.htm

[Rip]

This is the app you need (http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewSoftware?id=314296369&mt=8).

It won't work with the iPod music but will slow down other music.