First go to this hidden wall this way downstairs
as you can see. there's some various squares. triangles,two squares and a diamond. shoot them in order. diamond = 4, dual square = 4+4(8) and triangle = 3.
shoot this chest when you are near it,act 2 was been unlocked.

After hitting chest you will teleported to this library. it has fake wall go through red square that i marked. after that you will see an pattern of platforms. one of them has the correct was and this is how you do it after that you will get to another maze(dont forget to put teleporter. or else. you will die and literally do act 1 and pattern code again)do it like this (red square means that was secret way to get half life 2 room)
follow the blue arrows after you are done you will get to parkour with lava. put teleporter later. try to complete parkour. there's lava. it insta-kills you.

you completed it. now you can use music that appears on music panel. go to console.(noobs that dont know: go to settings, keyboard, advanced enable dev console) type this
bind k "+use"
it will binds your k with +use
+use is general action key. interaction will be used by this +use command.
select music after that. you done.
selecting random will gaves secret songs. and thats only way to get super secret song
NOTICE! tf_allow_player_use 1 will allow for +use to work. thanks N0where.
(there's large cooldown after selecting it. i hope you placed teleporter pal!)