For years I have been using "Amazing Slow Downer" by RoniMusic. Here is a demo:

Amazing Slow Downer Demo

Works great for changing tempo, key, looping. Unless the changes are extreme, you won't notice any warbling. Once you get a song set the way you like it (key, tempo), you can save the song as a .wav file. I have been extremely happy with this software.

For a while, Hal Leonard was featuring Amazing Slow Downer on their play along CDs to allow folks to vary the tempos and keys of their products.

Have fun!
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Randy

PA4X, SX900 (Baby Genos), Roland U-20, L1 Compact, Way 2 Many Saxes

"My computer beats me routinely at chess - but it's NO MATCH for me at kick boxing!"