Audio output problem on Amiko A11 Gold

I have problem with my box
I have only one optical out on this box
the audio is OK when I booting on Android and launch Kodi
On the other hand, when I booting on Coreelec, I have no sound
I have tested all the configuration in the settings, with no issue
where could the problem come from?
thanks a lot for your response

for information, the sound comes out correctly on the HDMI socket