From the Wold Wide Web Consortium (W3C): "HTML5 defines a new element which specifies a standard way to embed an audio file on a web page: the <audio> element." Here are some useful/interesting HTML5 audio/video players:

• MediaElement.js
• Audio.js
• jPlayer
• Universal HTML Audio Button ($5 paid license)
• SoundManager 2
• MooTools HTM5 Audio Player (MooTools library only)
• Speakker

List courtesy of Cat's Who Code!

And here's an interesting article of where things are at with HTML5 audio implementation: HTML5 Audio - The State of Play