Same issue here, but with the latest stable version (CoreELEC-Amlogic-ng.arm-9.2.0-Generic.img).
I have already tried going through the settings in Expert mode, as well as issuing via SSH:
echo "audio_off" > /sys/class/amhdmitx/amhdmitx0/config
but didn’t have any luck.
My box needs the rev. a dtb and was upgraded to the latest stock firmware (905P0) before installing CoreELEC, should that make a difference.
I would also like to take this opportunity to thank the developers that did such an amazing work bringing CoreELEC to this box, despite all the initial tribulations.