Is it possible to add background music (with or without video) to the login screen on a Mac?
That is a nice idea !
I have just seen that you can play sound with the program afplay, you can look how to use it here : https://osxdaily.com/2009/10/27/play-mp3s-and-other-music-from-the-command-line/ .
One way to do it could be at startup at the first login screen, you can look how to do it here : https://superuser.com/questions/245713/whats-the-easiest-way-to-have-a-script-run-at-boot-time-in-os-x .