some additional info:
Box has 2 sound "cards": ALC888 for Jack and optical, and NVidia for HDMI
after setting GUI, the /recalbox/share/system/.asoundrc contains correctly card=1 (nvidia) and selected device=7.
So my default alsa device has been set, "aplay some.wav" works!!
Unfortunately, this HDMI card has no mixer labelled "Master" (but some others!)
("amixer -cget Master" throws an error)
This is the problem, since ES looks especially for this name here:
(and fails with message in line 105)
So the final issue is, either to change the implementation, or to force some "Master" mixer creation?
It seems, that i needed to unmute only the correct device!!!
amixer -c1 sset 'IEC958',1 unmute
or via alsamixer does the trick for me.
Even ES complaining about failing mixer on startup, i have immediate sound 🙂
Now we just need to unmute the correct (sub)device based on settings....awww