Amazon app on Smart TV?
If so, that’s the problem. Well know for being buggy or just generally crap. I get audio drop outs on using DD5.1, but not if I change my TV to just plain 2 channel surround. Amazon forums are full of reports on this and they’ve never fixed it. Usual advice is to give up and get Amazon Fire stick.
It’s not the TV, as I have Plex app on it and that’s playing films and movies with no problem, 720 and 1080p in DD5.1 and DTS audio. I’d also made sure I had a higher spec processor in the Smart TV.
Thing that bugs me with Amazon also is old films can often be awful rips. Some looked like they were ripped off VHS!
Film selection on Prime is poor. TV is okay but it’s mainly exclusive and American centric content (except Grand Tour, which I realised was shit after a few episodes, but then Top Gear’s been shit for the last 10 years).