Sorry, can't give anybody anything, just a peer user.
Though the lack of full information at the time of release was a real disaster. Most certainly.
AVS4U supports it, you would think Premiere would. For now I am running the files through AVS4U's Audio Converter and then pulling the audio back into my Premiere project. Hard to believe that a much cheaper program has that capability, but Premiere does not.
I ran into this problem the other day and figured out a way that's not sketchy and doesn't cost you any extra $$
It does only work if you have a Mac, though.
1. Go to your App Store and Download SmartConverter (it's free and from the App Store so you know it's not sketch and exports the media directly to your iTunes)
2. Import the media with the AC3 audio
3. Choose the format you want to export the media to
there are a ton of options, .mp4, .mov, are probably your best bets
I personally exported the media as a .wav to be safe
4. Export your media
5. Import your new media into Premiere (sync audio if you do what I did) and BAM problem solved!
Hope this helps!
On my side I found an easy and other solution under Mac OS X High Sierra 10.13.4 :
1) Go to /Users/yourname/Library/Application support/QuickTime/
2) Remove the plugins found in the folder
It worked for me.
AC3MovieImport.component or Perian.component were responsible for the issue. One of the component is probably not made for High Sierra... Enjoy.