Hi Daniele! Thank you for your message. I read your “thread Screeneo U4 | Compact Ultra Short Throw - #180 by danbo “ and had the same problem. I connected my Apple TV via HDMI to the projector at the HDMI 1 input ( the input at the back of the projector, next to the 3.mm jack input ) and when I connect my RCA cable connected to a pjoneer apmlifier , the sound for 3/4 seconds can be heard and then it disappears and goes back to sound from inside the projector amp.
But the strange thing is that if I connect a pair of headphones and another pair of speakers the problem does not persist. I have also written to support but they have not replied yet. I then sent a PEC email to philips@legalmail.it to request an official solution from them.
Thank you for your guidance on using HDMI as an output as well, but I would not know how to then connect it to an audio amplifier with RCA-only inputs.