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.

Why is this happening? We're honestly not sure and I'd rather not speculate. We've reached out to Apple for comment but do not expect a reply.