I don't own a Sony Android TV, but what i would try is firstly go to My Account section of the NowTV App Home Screen Page and double check Surround Sound is selected under the Boost Pass.
If that doesn't help go into the Sony television sound menu settings and see if something needs to be changed to trigger surround sound.
I know @gavs82008 owns a Sony Android Smart TV, so perhaps he can offer further assistance on the matter before getting in touch with NowTV live chat.
Would have said try logging out of the NowTV App, unplug the power to the television for say 2 minutes, then try signing back into the App to see if that triggers the Boost Pass, but i doubt that will work.
Anything else in the HDMi chain such has a soundbar or AV Receiver that can be effecting it ?
One massively annoying thing I’ve noticed with Sony Android TV is that the sound settings regularly revert back to “TV speakers”, when this happens I have to change the sound back to “audio system”
When this happens the audio corrects itself and switches back to surround sound.
To be perfectly clear I use the Now TV app on the TV.
This "sound" issue happens regardless of what app I use. It even happens when I am playing the PS4 pro and leave it for a bit.
So I would press the settings button again on your remote and go to sound, scroll down to Audio System.
Sorry about the confusion the inbuilt Google Chromecast feature i mentioned wasn't discussing 5.1 surround sound where i was going off topic in a different direction (my bad).
Out of curiosity what is the make and model of your Soundbase and does it show on the soundbase display if you are receiving a 5.1 signal or stereo signal ?
I won't be much help, because i don't own a Sony Android TV.
On my Philips Android TV menu settings to get 5.1 surround sound to my AV Receiver via the optical out from the TV, i had to go into the Philips TV Advanced Sound Settings and from the three options of either Multichannel, Multichannel (bypass) or Stereo.
Multichannel without bypass (ARC) provides 5.1 dolby digital over the optical cable when watching Netflix, Amazon Prime App etc.
I have a Philips Android TV which doesn't have the Native NowTV App on it and sometime i use the inbuilt Google Chromecast feature on the television, but on my Philips TV the HDMI Enhanced setting sometimes screws up my 5.1 audio output on either my sound bar or AV Receiver.
Going off topic, i been meaning to ask a Sony Android user, if the native NowTV App on the television offers NowTV Bonus Sports Streams ?