I could not find a recent answer or update to this question in the previous discussions. If I read correctly, AA does not natively support importing mp4 video for audio sweetening and/or fixes. If a fix is available I missed the info and apologize in advance.
Is there a simple reliable plug-in to be had? I have some HDV video that could really benefit from some Audition help. I tried to import it from 5.5 Premiere Pro-to-Audition, but no luck. Making a copy of the video's audio in some audio format and then trying to sync it afterward defeats the concept of round trip workflow. Worse, it's incredibly inefficient. I've read long discussions about various baling wire and chewing gum patches which may or may not work, involving Nero and other programs.
Does Adobe have a fix? A recommended third party fix? If not, why not? HDV is still widely used and there are times when Premiere Pro could benefit from Audition help for run-and-gun video.
I'm surprised no one has yet answered this for you.
I think you'll find that if you download and install Apple/s free Quicktime, and restart Audition, that it will then read in mp4 files.
This is the normal way for Audition to work - it uses Quicktime's codecs for this.
You'll want to be aware that mp4 is a compressed format, so the usual cautions about re-saving in a compressed format apply.