My audio setting in recalbox.conf file:
------------ C - Audio ------------
Set the audio device (auto, hdmi, jack)
audio.device=[0:0] snd_rpi_hifiber - snd_rpi_hifiberry_dacplus + HiFiBerry DAC+ HiFi pcm512x-hifi-0
Set system volume (0..100)
audio.volume=0
Enable or disable system sounds in ES (0,1)
audio.bgmusic=1