I tried, but no luck. First, AST is off. I tried to switch it on and was forced into a speaker setup, but backed out. As a result, only the center speaker was active – real men use mono! I suppose I could have gone through the whole speaker setup, but didn't bother. Set the AST back to off, and the speakers came back.
Next I went into speaker setting. I have Beo6, but the functionality should be the same. Sorry, Speaker 3 didn't make any audible difference. The sound from the back speakers (= nothing) is added to the front speakers, but the sound from the center (= nothing) is left alone. I can choose Speaker 1 and have a nice mono for speech, but too thin for anything else.
Technical support at Viaplay has sent a polite regret for the situation and passed the problem to the engineers.
In the meantime I can choose between thin mono (Speaker 1), talking walls (other settings) and flickering images (Mac, not PS4).
Thanks for all your help!
Sigmund