I succeded just after writing the post above.
This time kick31.rom was in roms and not in bios.

Thanks for helping!

I have the same problem did you find any solution ?

Solution: the problem was that it's NOT /boot/recalbox-boot.conf that should modified but this: https://github.com/recalbox/recalbox-os/wiki/recalbox.conf-(EN)

Ihad the same problem. As @LBR59 said, the video is located here:
If you are on Linux (but I can imagine ffmpeg tools are also available on windows), this simple command line will create "outputfile.mp4" with the sound lowered:
ffmpeg -i recalboxintro.mp4 -vcodec copy -strict -2 -af "volume=-15dB" outputfile.mp4
Hope it may help

Glory to @sevenx and @Substring \o/

4.1 will be awesome :)

@Substring I just checked the boxes for the OSes I wanted, then clicked install. It was connected to the internet then.

@Substring here you go https://www.dropbox.com/s/v68uy2p7xw9vbkh/recalbox-support-20170220181908.tar.gz?dl=0

@Sling Hi

It looks like you've already paired your pi. But let's doo it from 0 then. Unpair devices inside GFE then run :

cd /recalbox/scripts/moonlight/ ./Moonlight.sh clean ./Moonlight.sh pair ./Moonlight.sh init

@blackmagik

Create below file and put in your command
/recalbox/share/system/custom.sh

Make it executable
chmod +x custom.sh

/recalbox/share/system/custom.sh (if exists) is ran at boot.