I have a new Pi 4 running the latest LibreElec. It is connected to my Samsung TV via HDMI and to my AVR via SPDIF via a HiFi Berry Digi+ card. There are two other Pis connected to the TV via HDMI... a Pi 3 running OSMC and a Pi 2 running RetroPie. I use the TV's remote to switch between them and control them via CEC.
When anything is playing on the Pi 4 with Libre, if I switch to one of the other PIs via CEC, the audio stops. This is expected for audio via HDMI but should not be for audio via the HiFi Berry card to the AVR, the audio should just keep going out, allowing audio to the AVR whilst other devices send audio via HDMI. (ie music playing from Pi4 to AVR while playing movie to TV from Pi 3).
Is there some setting in LIbre that stops audio on all ports when switching devices via CEC? This doesn't happen with OSMC
ANSWER: Ahh, found it. There's a setting in the CEC Adapter add-on to pause the Pi when switching devices. I turned that off and all is good now.