The HTML5 standard only says something like "this is the HTML5 audio element and this is the structure, now go ahead and implement whatever audio formats and sources you like".
Apparently my information is a bit outdated. I last looked at the HTML5 audio element 2 or so years ago and back then it was a catastrophe because every browser supported different codecs.Here's some up to date information from the Mozilla Developer Network:
The MP4 container format with the H.264 video codec and the AAC audio codec is natively supported by desktop/mobile Internet Explorer, Safari and Chrome, but Chromium and Opera do not support the format. IE and Chrome also support the MP3 audio codec in the MP4 container, but Safari does not. Firefox/Firefox for Android/Firefox OS supports the format in some cases, but only when a third-party decoder is available, and the device hardware can handle the profile used to encode the MP4.