We've been using SoundManager 2 on quizlet.com for playing audio clips. We're not just playing long music clips in the background, so it's essential to have all the precise APIs for preloading, playing, and firing callbacks when sound is done playing. It handles all the browser deficiencies very elegantly, either with HTML5 or flash. Also, it's just 10k of js (gzipped) so it's pretty lightweight.
I'm using sound manager on http://hackernewsradio.com as well. Right now I am just forcing flash. I had too many problems with html5 audio.
I really wanted speex encoded audio but that is not an option. If I remember correctly Opera handled streaming speex/ogg like a champ. Firefox may have worked... I can't remember. Everything else sucked.
link: http://www.schillmania.com/projects/soundmanager2/