I watch sky sports through the app on my mobile device and every time I'm watching Formula 1 live there is no sound. I have to close and reopen the app 3+ times before the audio works.
Watching on demand items the sound works instantly everytime.
Have you tried to delete and reinstall the app?
Does it happen with any other live channels?
Is this over your home Wi-fi or mobile data?
Also reported on this linked thread below.
I personally don't use my smartphone when watching NOW Sports so can't confirm if the issue is more widespread or on certain smartphone models.