According to the how-to's (the good ones anyway) we should do it as felgall mentioned above.
AAC is just supposedly a better version of mp3, but I didn't make this file and I don't own any converters. I'm solely testing the HTML part here.
All I'm looking at is making this work on chromium (could this be different from chrome?) who should work with the code here http://stommepoes.nl/downloadtest.html (the picture is a different thing I wrote for someone on SO, so I'm only referring to the audio controls underneath).
There must be something wrong with that, yet it matches the most basic setups listed by places like HTML5doctor (so, without all the fallbacks I shouldn't need anyway for the browsers I'm testing in).
Honestly the mp3 came from work and I'm thinking of telling them to just use Flash anyway, since old IE is indeed an issue for us. But I was hoping to see fancy HTML5 audio working out of the box first.