We ran a few tests and this is what we found: most files arrive bit-for-bit identical compared to the files that were sent. But sure enough, MP3 files were downgraded considerably. The 5.12 MB stereo 228 kbps file I bought from Amazon was downgraded to 1.4 MB mono 62.4 kbps on receipt after transfer through Messages.
Messages compacts MP3 audio | TUAW – The Unofficial Apple Weblog
