The Beebox-S 7100U does have 2x HDMI ports. These HDMI ports always cause audio dropouts when using LibreElec 11. I also never had audio with DTS-HD High Res. I did not find a solution for the HDMI ports.
So I started tests with the display port. I tried several Displayport-HDMI adapters. Unfortunately also always only problems. Until I found the BENFEI Active DisplayPort to HDMI Adapter. Since I use this adapter, I currently have no more audio problems.