Supported audio formats
The HTML5 spec is a bit vague on proposing recommended audio formats (remember that it's still a draft). While at some point, Vorbis format was supposed to be a sought after standard due to it being free of patents and such, the popularity of formats such as MP3 might actually make them the official standard. For the time being, while Chrome is now leading the browser charts and it supports a wide variety of formats (Ogg Vorbis, WAVs, MP3, AAC, WebM, and others), it is probably best to provide audio files in both Vorbis as well as MP3 or AAC to make sure all browsers support playing the audio content.
Browsers support matrix for the <audio>
field formats provided by Wikipedia (http://en.wikipedia.org/wiki/HTML5_Audio), as shown in the following screenshot: