Hi Dean Welcome to the fourm
Try these steps to help with the issue you are having;
Start by uninstalling the NOW TV app
Now that you have the latest update, your sound should be back in sync.
Thanks The Nowtv Team
I have just bought NOW TV and I'm having the same issue. Sound is fine on the Xbox, even to the point where a programme is started in the NOW TV app, when there is no sound from there on. Going back to the Xbox and there's then no sound. Have to restart the Xbox to get sound back. I've followed the instructions provided by Nadine-K but this has not solved the issue.
I noticed that the Optional Media Update is automatically installed, even after reinstalling the NOW TV app.
Any guidance appreciated!