iTunes without audio on some movies, speakers are fine, latest updates on - Solution Found

No previous answers here worked for me but my husband found a solution that worked. In the movie file itself, hit open, play, etc. so that the movie is playing. Click on the button that controls subtitles (dialogue box next to scenes list button). Look for the option "Prefer AC3 Passthrough". If it is checked/selected, uncheck/unselect it. Turn it off. My audio instantly worked again.

Apparently, Prefer AC3 Passthrough is an option for devices that are able to do surround-sound. Checking it means you prefer that. If your device or PC isn't set up for that, having this checked will turn off your normal options for regular audio and prevent audio from working. I don't know why this suggestion wasn't on the other answers and I've been trying to fix this for a long time (months) and no Apple tech I called could fix it, either.