Have a look at this link for troubleshooting.
If that fails set the box back to factory settings.
I hooked up my NOW Freeview Smart Box yesterday (which i don't use anymore) but noticed annoying random audio drop outs when watching a couple of things with my various NOW TV Memberships.
I seemed to have resolved it by going into the NOW Box settings and selecting Stereo, where the audio dropouts for me on the NOW Freeview Smart appears not to be happening any more from my couple hours of testing.
Obviously if you are using a 5.1 surround sound system with the NOW Smart Box, then this above workaround would be of no help.
From memory i went into Settings > Audio > Audio Mode > changed from Auto and ticked Stereo.
Left all other audio settings untouched from what i can remember.
It might not work in your setup, but from my couple of hours of testing on the NOW Freeview Smart box it seems to be working over my TV inbuilt speakers when watching NOW Entertainment & NOW Cinema, where i haven't noticed any obvious audio dropouts since the changes made.