media player & changing song speed for Mac?

Take a look in the app store for Capo. You have to pay for it, but it's completely worth it... you can slow down, change pitch, loop, apply filters to remove the guitar.

The best feature though is the spectrogram it creates of the music. If you click within the spectrogram, it adds that note to the tab at the bottom of the screen. Makes it really easy to tab new tracks.

You can get a demo copy here:
http://supermegaultragroovy.com/products/Capo/

 

Garage band might do what you need. I know it can adjust the timing of a recording, slow it down and speed it it.

I think the way to use it is to impoort the mp3 to an audio track then change the tempo, should "just work"

You can adjust the pitch too if you like a few seitones up or down