We found the issue
Somehow when the volume slider of MP (in Open Volume Mixer) is manually set at bottom or a tool set it at bottom (didn't find the cause exactly), the volume never goes up after that and we need to manually higher it and after all works correctly
Issue :
So we need to higher it and after all will works :
I had this same problem in 1.18 final - drove me crazy! I couldn't track down the problem and ended up installing Kodi as the client, but your solution fixed it - thank you. I have no idea how it got into that state though - I certainly never used the audio mixer to adjust the volume manually.