It sounds odd to me too, And even if it isn't supposed to be able to do it, it's so odd.
A friend of mine gave me a pair of host files to see what I thought of his game, and I immediately heard a song, and knew it had to be ogg, or mp3. I raised the question to him, How'd you include the mp3, through the browser? He said there was no mp3 in there, but a midi. I knew this was no midi song, so I challenged him, then realized what it is he had done, he renamed the extension from mp3 to mid. And by some oddity, it actually played the mp3 song in Byond.
Has this always been the case? Did BYOND add detection of the mp3, and have it access a hidden mp3 player? Or is this merely a isolated incident that probably won't work with all mp3's ?