Hi everybody,
I think I fixed my problem.
It turned out my device
was not the on-board sound, but the Hifiberry.
If you use something like
you get more information about your sound devices.
By accident I checked sound is played properly after loading the modules using
As I don't want to load the modules manually, I put the load command into /storage/.config/autostart.sh.
As the last step I had to choose "Bluetooth" as the default audio device in Libreelec. With this setting I'm now able to stream audio from a mobile device via A2DP and have LE played another sound at the same time.
Hope this helps,
Christoph