There are only 4, you unlock them by full combo'ing each of the four NB RANGER songs in freestyle mode. You can do each of them on 4 Button - easy mode to make the whole thing more achievable. Some of these songs may need to be unlocked before you can play them but none of them are late level or challenge unlocks so they will come relatively soon
You have to watch each video in full before the achievement will pop, you can't quit part way though. Collection - Gallery - Videos is where you can find them to view
The songs aren't very hard but may be challenging to newcomers who are still transitioning to Hard mode and similar intensities. NB RANGER - M2U is the hardest rated song of the bunch and will be what most people have trouble clearing. I turned my note speed effect up a little bit (to 3.8) didn't worry about precision too much to make it though.
All 4 of them unlock based on Player Level. Player Level 30 is the highest.
You can also play the songs in Air, Open Match, or Ladder Match.
The important part is that you must MAX COMBO the songs in order to unlock the music videos in Collection.
Something to note is that if you got the storyboard version of NB RANGER - Virgin Force you won't be able to get the achievement unless you get the normal version although I'm not sure if you need both versions as I got the storyboard version first and watched the 4 before getting the normal version
CREDITS: Soundteqs, Shadow Kisuragi and Kazamaki for the information
https://www.trueachievements.com/gamer/Soundteqshttps://www.trueachievements.com/gamer/Shadow+Kisuragihttps://www.trueachievements.com/gamer/Kazamaki-5143