Is it good idea to make FreeBSD videos on YouTube ?
Of course it is and I can't see any reason why it would be a bad idea if you do it because you like the OS. I mean it can't hurt, promoting an OS through videos to show what could be done with it can only help.
Regarding the music or anything artistically related IMO one should do his own thing the way he likes it, and if it's different let it be because you won't make everybody happy whatever you do.
Relaxing music yes why not, plenty of them on YT.
Personally as hobbyist I enjoy discover things through videos(YT or asciinema), I don't take it as a full tutorial but more as a first step in I am fully aware that it's obviously impossible to be as complete as a documentation can be.
There are some youtubers very qualified and really good at what they do unfortunately there are also a lot of bad videos but it's also true for blogs so ... it is the same thing everywhere on internet nothing new.
Last days I tried to learn things about jails so I looked at 'bastillebsd' and 'cbsd' videos both made by their author. That way I can watch them in action and see the basics.
But If you are an IT guy I guess you won't look over YouTube for a tutorial, because excepted specific channels you probably are not the target anyway.