Hi Miki,
sorry I missed your message and today I wrote about the same issue in the general discussion (see here Screeneo U4 | Compact Ultra Short Throw - #180 by danbo).
As you can read I had a similar issue. I tried already to get in touch via email to the Screeneo customer service, but I have not yet received a feedback from them.
Update with respect to this morning is that I succeeded to get permanent out out from my stereo system using HDMI instead of the 3.5 mm jack. These are the connections I set up: HDMI connection between my laptop and the projector (HDMI 1 port) + HDMI connection between the projector and my stereo system (HDMI 2 port, since this is the only one that supports ARC feature!).
Still, I would like to understand the reason behind the issue with the 3.5 mm jack audio output.
Cheers,
Daniele.