He uses iframe tags with a Youtube URL's width/height set to zero. You can see for yourself by hitting f12 on his site - otherwise here is the answer on StackOverflow: https://stackoverflow.com/a/31108751
There used to be music here, but ever since browsers have changed media autoplay policies, it is now impossible to play any sound in the "Go Visible" pages. Instead you can find the sounds here: https://www.youtube.com/channel/UCY2DVc6iyK5_1BpIjdGmMvA
or you can download it from the site itself.
SEL is great! I also recommend Future Boy Conan and Great Teacher Onizuka!