.ogg

The Ogg Vorbis file format is a open patent-free music encoding file format. It is generally used as an alternative to .mp3. Many games and other products use this as it means it's free for them to use. Many people say that Ogg files sound better. It's video brother is Theora

How it works
This works in a similar way to .mp3 but it works a bit better (dunno how).