Please disregard the big tick on my post re the sound and vision sync on Discovery+. It's not solved at all! I just didn't realise the significance of accepting answers.
To summarise, sound arrives on my Now box fractionally before the picture. Doesn't happen with any other app. Box is always unplugged after use, and router usually reset before using Now box. Any sound settings I can find have been experimented with, both on box and television. And I've uninstalled and reinstalled the app.