Fair Use gives you roughly 30 second clips
The container (mp3) doesn't matter so much as the content (The song itself)
Generally speaking, if you're not selling your game, you're fine to use whatever you want art/sound asset-wise from Nintendo, and the worst you could get is a cease and desist letter
If you try to use like Guns N' Roses in your game you're not selling? You're probably fine there too to be honest. Again, before they would take you to court they'd send you a letter asking you to stop.
But don't try to sell ANYTHING that you or your team doesn't create yourself.
Hope that helps some