In a previous tip I showed how to slow down a movie.
This technique does not seem to work for mp3s.
For one purpose I needed to slow down a sound track about 10%. For this I used Audacity which works really nicely.
Open the file, and then from the Analyze menu select 'Change Tempo'. Put in a percentage. As a guide if you don't want the ear to 'hear' that things are slowed down / speeded up you cannot change more than about 10% or 12%, your mileage will vary.
Since Audacity can open various file types eg .wav .aif and even .mov, and then export to mp3 if you get the free LAME plugin, this technique works well for most sound sources.