Okay, I've just tested this and it works (to an extent) but for some reason when I enter the text, and hit post - it shows up the code like this in the actual post-
[mp3]http://www.site.com/audio/sound.mp3[/mp3]
After a refresh it's still the same. But if you click "Edit post", change nothing and then hit "Save" it's fine and it shows.
Quite annoying

Do you know why or of another way we can do this?