Hi, I still use these little boxes quite regularly, and part of the ritual I must go through whenever I turn them on is to hold one of the buttons and turn one of the knobs all the way up, then un-turn the knob I turned to the setting I wanted it to be on previously 
I see in the firmware that there is a AUDIO_OUTPUT_GAIN constant that if raised to 127 (I think, or whatever the maximum is), then that would solve my problems and I’d never need to set the output volume manually again. But I am out of my depth compiling and rolling my own firmware, don’t have an ST Link and don’t have a machine set up for compiling firmware. I would rather send someone some cash to someone who knows what they are doing to do it for me.
Please let me know if you are interested! Happy to work out a price etc if I see anyone might be keen