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
