Sigil has the option to insert a file and it has Audio and Video folders and when adding an MP3 it adds the code for it, '<audio controls="controls" src="../Audio/test.mp3"></audio>' and adds the file into the OPF. It even shows Audio and Video folders when you start Sigil thus suggesting it has some support for them.
At least if it adds MP3 have it add correct application type. No worries though, I can have my ePUB2 to ePUB3 converter 'fix' the issue automatically on conversion. I end up with a fully compatible and validatable ePUB3.
As for why one wants to add audio or video to an ePUB in the first place is something that might be useful is some cases. An instruction video in an ePUB is one of the examples.
I know Sigil is ePUB2 only, I know ePUB2 doesn't support audio
That is why I made my convertor to convert the ePUB2 to a valid ePUB3.