The reason is mp3 is a non-free patented technology, and would people like to pay to get a browser with mp3 audio elements?
it might change in the future (if it’s not already the case, I don’t use much mp3 so I didn’t test this)
(by the way ogg vorbis is a more efficient encoding format).
I tried also to play some sounds with Vorple, but it doesn’t work as expected (either in firefox or chromium):
When play begins:
play ogg sound file "file01.ogg".