I haven't seen anybody on the NOW Community Forum or over on NOW Twitter report this.
Unfortunately i haven't got a Samsung Smart TV with the inbuilt native NOW App to check.
I tried Sky News on my NOW box and Roku Player and both run Sky News fine with regards to both picture and sound.
Can't really suggest much apart from temporary unplug the power to your television and Router, maybe a fresh NOW App download.
There is about 3 different ways to access the live Sky News TV Guide from the NOW App, maybe try all three methods.
Apart from that get in touch with NOW live chat to report the problem, though it does sound an isolated case (unless more Samsung Smart TV owners come forward).
Just to echo the sound advice from @schnapps if you are unable to delete the NOW app, I would suggest to clear the app cache.
Worst case scenario to set the TV back to factory settings.
Two other people have reported the same issue over on NOW Twitter in the last 24 hours with a Samsung TV.
Unfortunately reading the Tweets between NOW and the two people who have reported it, they still can't get sound from Sky News via the live streaming feed off the TV Guide inside the App.