My friend Little Andy (andy1) managed to figure out how to get a custom music trigger working so all credit goes to him.

I'm not sure if there is any other ways of using custom music scripts, however i find this simple and easy.

If you understand how to create a trigger, this can be skipped.


Then place it down where you want it.
Activation can be set to whatever side you want, preferably Any Player so if any players enters the trigger, it activates.

Make sure to change the area that the players enters in
Transformations
Size
and mess around until you get the desired size.

IMPORTANT
Set it to SERVER SIDE and NON-REPEATABLE.
if you do not turn these on, whenever players enter the trigger, the song will play again.

Next Up, Music.

Music files must first be in .ogg and not .mp4
go to your where your mission file is in so:
C:\Users\[Username]\Documents\Arma 3 - Other Profiles\[Your arma 3 name]\mpmissions\[Mission name]
and create a Music folder, in which you place your music files like so

Next you'll need a description.ext in your mission folder.
to do so, create a .txt file, and edit the name so it's ".ext" instead.




Now edit it with notepad or Notepad++ and write in:
class CfgMusic
{
tracks[]={};
sounds[]={01};
class 01
{
name = "01";
sound[] = {"music\[insert name of .ogg file]", db+0, 1.0};
};
}

it's easier in my opinion to number them so name will go 01, 02, 03 etc.
remember, if you add more songs, remember to add name to the "sounds[]={01, etc};" so it would be {01,02,03}

an example of mine.

This part is really easy.

Go to the trigger you had placed

Keep Condition to "this"

In the On Activation, write:
["[Song name]"] remoteExecCall ["playMusic", 0, true];

An example is
["02"] remoteExecCall ["playMusic", 0, true];

SET THE TRIGGER TO SERVER ONLY!!!
this stops it from playing multiple times whenever someone enters the trigger.

Place a unit, control it, and walk in.
If music plays you did it!
if not, check over everything and make sure nothing is missing, etc.