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.