Create a file called description.ext in your mission's folder:
class CfgMusic
{
tracks[]= {};
class Mus01
{
name = "Music 01";
sound[] = {\music\01.ogg, db+0, 1.0};
};
};
In this example, the music file must be located in the folder music inside the mission's fgolder and called 01.ogg. Obviously you can change that. You can change also the name in the "class" line and the text in the "name" line.
Now if you want to play the music, type in a script / trigger / waypoint
playmusic "Mus01"
Or whatever name you put.