Sorry for slow response...
Finally, I solved the audio issue.
Explanation:
My Odroid C2 is connected to my TV (Pana Plasma), and my TV is connected to a DAC via optical cable which is connected to my stereo amplifier.
I do that cause I cannot connect directly my Odroid to my DAC (usb, spdif, optical input) or my stereo amplifier (no digital input).
Then I tried the following:
1) Audio setting\output config as fixed, and I tried all the Freq levels from 192KHz down to 44KHz.
And then the Alsa HDMI setting is finally working but only at 44KHz (on a movie using DTS)...
So the pb comes from a limitation of my hardware (HDMI ports from my TV should be limited), and not from Kodi/LibreELEC.
2) I also tried Audio setting\Best match, and Passthrough activated and all sub-options (from Passthrough) deactivated (considering my stereo is not able to "read" other things than stereo...).
But not working...: still got Kodi system sounds, but no sound when playing a movie.
3) Finally I kept Alsa Analog option cause I find the sound has better quality than Alsa/HDMI/output fixed up to 44KHz.
So it works.
But I don't understand what Alsa Analog means considering my Odroid is only plugged with a HDMI cable...
Any thoughts?