What are your video and audio codecs?
Here it's hevc and aac.
Even with a good signal quality, after switching to another TV channel, sometimes the audio is not detected properly. Very annoying and long lasting bug. Usually there are 2 options selectable in the audio stream settings:
Czech - AAC - 2 channels (1/2)
Czech - AAC - 1 channels (2/2)
Where the first one is working option which is usually selected automatically. The second option (Czech - AAC - 1 channels (2/2)) is producing silent only.
And sometimes after switching the channel, there's only the second (thus not working) option is there or there's even no audio stream option available.
Switching to another channel and back usually fixes the issue .