It's barely a situation; that's why no new mkv is getting released. If you just want to be able to listen to the ac3 with the movie and are using a Windows laptop connected to a TV, you could just add the audio you want in the dub field of the Open File dialog of Media Player Classic Home Cinema (open with Ctrl + O):
A more permanent solution not dependent on what playback program you use would be to use MKVToolNix to mux the new audio file in and leave out the old one:
