The purpose/advantage of HTML5 is supposedly to have everything you need in the browser without further downloading required (apart from the game assets themselves).
Adding an audio plugin would defeat this purpose.
Maybe, in the vein of what Tom said previously, launching a kind of petition to make pressure on the browsers company so they :
* Implement the missing formats
* Agree on some audio standard that all browsers would implement
I'm not sure there's so much money to be made out of audio and video formats (even if some companies have made) as it is involved with another standard (HTML5). This defeats the profit of standardisation and won't help a browser to win over the others. On the contrary, I'd dare say that the first browser to have the most formats supported might be a winner.
Also, by not providing a decent and compatible single audio format those browsers push the unnecessary use of more internet bandwith (not having a single audio format supported leads to the multiplication of audio files (to cover the clients' browser audio support possibilities))
bigcatrik, I don't know if it the case for this peculiar website but I've encountered html5 libraries whose audio support was based on flash (counting on the major deployment that flash is still enjoying).