I don't own a Samsung smart TV and can only suggest some basic stuff to try, like switch the power off to your TV and Router for about 30 secs.
If that doesn't work, maybe try removing the NowTV App, then re install the App again from your TV Apps Store.
Should you be using an external playback device such has a nowtv box etc, power down both the box and TV and temporarily disconnect the HDMI cable at both ends or alternatively try a different HDMI port on your TV.
Did you find out how to get sound in the end?
I'm having the same problem now!
Hi, I’ve just purchased sky day pass on a Samsung tv and there is no sound. Picture is perfect but no sound at all! Please help before Utd / Liverpool game kicks off😫
A couple of Samsung Smart TV owners had success with getting the sound back on the NowTV App on their TV's by unplugging the power socket of their TV from the mains outlet for about a minute.
Might be worth a go if you haven't tried this already.