Did the ePUB2 validates correctly with Flightcrew and ePUBCheck before converting to ePUB3? If so, please report these errors also in the ePUB3 plugin thread. I am wondering this, since adding an audio file should not give those errors in the opf at all.
An image must have the property for an alternative title in it. So, for example, <img alt="description of image" src="<image>" />. To be honest, most people use
alt=""...
I do think we need to see the OPF to give a better answer.