Arg so frustrating!
I nosed around and found this, link to web page at the end:
The PlayStation 3 has its own audio configuration that must be set up correctly for your TV to accept the sound the console transmits. To change these settings, you need to first know what type of setup you have based on the connections you use on the back of your PS3. There are three choices: "HDMI OUT," "AV MULTI OUT" and "Digital Out." You might even use "Digital Out" in addition to one of the others.
https://ourpastimes.com/no-sound-when-using-netflix-on-ps3-12597815.html