I have the NOW TV app on my Samsung TV. I was watching the Grand Slam of Darts a few weeks ago on Sky Sports and all through the tournament the audio was intermittently dropping out. Every few minutes the sound just goes off for a few seconds and comes back on.
I havent watched sky since the tournament finished but tonight I have been watching the darts again - The World Championships and the sound issue is still occurring. This issue doesn't occur on other Now TV channels such as Sky Atlantic.
Try deleting the cache from the app.
Try unplugging your TV for a few minutes and at the same time your router.
If that fails you need to report to NOW via live chat. Make sure to click “chat online” within the green box, don’t hit “get help quicker” as this is a bot.
Or email support using the address in the below picture.
Happening to me I've done everything mentioned and still no joy. Darts is on main channel now so I can watch it again. I've been in touch on chat maybe if happens to lots of people they'll provide a fix. Fingers crossed!