It seems to me that it's using Quicktime's midi engine. The focus is not on sound synthesis at all, it's on composition. When you download the composition it's sent to you as a midi file. With that you can easily assign better sounds than the ones provided by Quicktime.
Mind you the Terms of Use are brutal and you're basically not supposed to do anything with that midi file anyways. Pretty much any creative use of it could be considered derivative works.
I first heard about WolframTones on The website of the physicist turned mathematician John Baez. He composed a pretty decent album using WolframTunes called Treq Lila: http://math.ucr.edu/home/baez/music/treq_lila/
This is pretty cool but I am a bit disappointed. The classical generator doesn't generate any recognizable genre of classical music that I've ever heard. I managed to get one that sounded like a phase-cycle piece without actually having any phasing.
(also the creator of most music you remember from Warner Bros. cartoons, and inventor of the electronium, a totally analog machine that composed its own tunes, customizable by the "performer")
Amazing. Whoever didn't see it yet should watch Stephen Wolfram's talk about computation: http://www.ted.com/talks/stephen_wolfram_computing_a_theory_...