There is no open source for MP3. MP3 encoder/decoder require license fee. If you pay for that, you can get a C or C++ source code from the licenseer. I don't know if you develope for your own personal use would require a license or not. But you can check at the following site: www.mp3licensing.com
Series 60 from Feature pack 1 has been supporting MP3 audio file playing, so for a player, you can use standard player classes. see sounds example for more information of how to play files in series 60 environment.