Robust watermarking in change format
Watermarking and steganography are very related to each other. While steganography is interested in capacity and invisibility, watermarking is to provide robustness to protect hidden data against any attacking methods like compressing, cropping, noise addition etc. LSB Coding, Phase Coding, Spread Spectrum, Echo Hiding are basic methods you should search about.

I suggest you to read these articles for the basics:


Audio Watermarking Techniques, Hyoung Joong Kim
Robust and Reliable Audio Watermarking Based on Dynamic Phase Coding and Error Control Coding


First article is very helpful to start. I advice you to try Spread Spectrum method, it was very robust against mp3 compression as I have coded myself.  I had answered my own question here trying to explain steps of Direct Sequence Spread Spectrum Method to embed data inside audio files. It may also help you.