MP3 decoding requires a lib unless you're a masochist (it's an incredibly complex format).
If you want to do this "by hand" you can, but it would be a lot of work and there wouldn't be much point.
There are many audio streaming libs that can play mp3 and other popular formats (like the superior .ogg ;D). If you use such a lib, this is a very simple thing to do.
What do you want this for? Is it for use in a game? Is it for a media player? How it will be used will make a difference in the appropriate lib to use.