To clarify, I think this is how you are setup =
PS4 HDMI out port --> retail HDMI to HDMI cable --> U2718Q HDMI in port
GTX 1080Ti DP out port --> U2718Q DP to mDP cable --> U2718Q mDP in port
Concerning audio via the U2718Q. The U2718Q does not control or manipulate the audio entering it via HDMI/DP/mDP. The U2718Q simply passes the audio through its bottom Audio-Line out port. The act of rebooting the U2718Q is forcing the PS4 to re-transmit its video and audio handshake.
I think you will need to post this PS4 audio issue on the PS4 Forums. See if other PS4 owners can contribute some troubleshooting steps.
My PC is turned off whenever I was playing my PS4.
I first thought It's the problem of PS4, but none of my PS4 friends has the same problem.
I just tried, I use HDMI prot alone to transmit my PC video and audio to U2718Q, dis connect all other video cables, then connect my speakers to auxiliary port, It has the EXACT SAME problem, Sound is burr and harse, multiple reboots solve this, reappears after a while
You should still post the issue on the PS4 Forum.
"but none of my PS4 friends has the same problem."
And were those friends also using a 2018 4K monitor?
Get an exchange. Test the replacement before returning any monitor to Dell. If both monitors have the same issue, it is highly unlikely that both monitors are defective. Look to the devices, drivers, cabling, etc.