Are you using the HD Homerun Flex4K? If so, and tuning to ATSC 3.0 channels, audio will not work because there is no support currently for the audio it produces. Your only recourse is to make sure to tune to ATSC 1.0 channels.
The HDHomerun App can play the sound though, but Plex can’t. See this thread:
If you are using the Flex4K, not sure what your problem is.
