I take my lectures and import them into Audacity (open source software). I increase the Tempo to 65, then I resave it to MP3, and listen to it in Media Player. Why WMP? well, I agree with others that it has a really decent play speed feature, and even lectures sped up to 65% could be sped up more, I mean sometimes the professor is just saying something you know, or don't care about, just listen to it in super fast speed mode.
Have fun with my trade secret
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WMP:
CTRL + SHIFT + G = Play Faster
CTRL + SHIFT + N = Normal Speed
CTRL + SHIFT + S = Play Slower
You could turn the audio files into AAC files using Itunes, then delete them from itunes, rename the file on your harddrive from m2a to m2b, reimport them into Itunes... then onto your ipod. Now they're considered Audiobooks and can be played faster or slower. Frankly, not worth the time. You're better of just listening to it at normal speed. I recommend you do the Audacity step and then import them into itunes->ipod.