I made a little HTML game a year and a half ago that made some basic use of HTML5 audio, and it worked well enough, but it was very finicky, but I could see HTML5 audio eventually being useful once it matured.
Recently, I reprogrammed an old XNA game I wrote into HTML5 (I never want to go near XNA coding or the XNA community ever again), and I was hoping to program a much more extensive audio engine for the game. Unfortunately, I found HTML5 audio support to be just as finicky as before. Apparently the iPhone/iPad can't play more than one audio stream at once, meaning you can't play music and sound effects at the same time, so it's not possible to make HTML5 games that work well on the iPhone and iPad (basically, all of Apple's talk about HTML5 being superior to Flash on the iPhone and iPad was just propaganda and not reality). Firefox doesn't play short audio files, so the bips, clicks, and other sounds for button presses won't play. Chrome (and possibly Safari too) will not let you play an audio file more than once unless you serve the file from certain types of web servers. The audio in Chrome just sort of dies out after a while anyway, so you don't get any sound after a while. The Internet Explorer 9 Release Candidate seems to only support mp3 files for audio, and using mp3 files in games requires the payment of a licensing fee of $2500 to the mp3 licensing guys. So basically, it's not possible to build a very basic, reliable sound engine for HTML5 games right now.
This struck me as strange though because there are plenty of HTML5 demos and games that have extensive sound support. How can they pull off reliable sound while I can't?
Perhaps by this time next year, it will be possible to build a pure HTML5 game with good sound and graphics. I haven't used Firefox 4 yet, but I have high hopes for it. Until then, I think Flash is still the only viable platform for online games.