I have a solution
be sure that you recalbox is connected to internet (wifi).
Run a command dos
type :
ssh recalbox -l root
(-l set the login)
When SSH ask you to enter password type
recalboxroot
Now you can set the audio.
Type F6 to select sound output.
Choose heaphone (or your output)
Now you see the volum bar of you headphone output.
Put it to 100% to solve your problem