MP3Stego
From Just Solve the File Format Problem
MP3Stego (also MP3Steno) by Fabien A. P. Petitcolas hides short files inside MP3 files.
Embedding the data takes place during the encoding phase, so it can't directly modify an existing mp3. It would have to be decoded to WAV and re-encoded.
Hidden data is compressed with ZLib and encrypted with Triple DES