I have found a way but it only works when the game is launched outside Oculus Home, for example launching from Steam. It is just to set speakers as default in playback devices and to set stereo mix as default in recording devices but then in Properties of stereo mix send the sound to rift headsets.
But anybody has been luck enough to find a way for content launched from Oculus Home?
OK, how stupid I have been. It is super easy. I forget to check settings for this. There is a setting in Oculus Home to set VR audio output. It is possible to switch there from headsets to Windows default. Once changed to Windows default with previous trick configured and now I can hear both headsets and speakers.
This way you can do demos with image and audio to an audience.