HTML 5 is advance version of HTML 4. In HTML 5 some new tags are added and some old tags that were unnecessary were removed. As there are too many formats of Audio for example mp3, acc, mp4, etc. Now the question is which audio format is not supported by HTML 5. Well there is nothing like which audio format is not supported by the HTML 5, HTML 5 supports all types of Audio format but sometimes we see that some format is working in one browser and not in other browsers. It is the problem of browser not HTML 5, Why is this...
A browser is a software in which we run a web page. There is coding for which type of audio format will be supported by that browser and which type not. If a browser is not coded to support some audio format then that format will not work in that browser, so it totally depends on the browser you use. Most browsers support all kinds of audio formats but some browsers don't. for example, chrome supports all audio formats whereas Internet Explorer supports only mp3 and mp4 format audio.
For more details, you can read an article on Wikipedia - Read Here