Ok,first you need to make sure your music is in OGG format.i dont know of any free convertors only trials,a good one is the dbpoweramp converter.Now to put the music in just paste this code into the description.ext.Also make sure there is a music folder inside your mission.Obviously you would change the names here to the file name of your song.

class CfgMusic
{
tracks[]={filename};
class filename
{
name = "filename";
sound[] = {"\music\filename.ogg", db+0, 1.0};
};
};


The track name will appear in the tracks list ingame.What i did for my mission was put a trigger at the opfor spawn so when the game started and someone spawned the music went off.So you can make a trigger right outside the base that activates when a blufor unit gets to it Or you can make it so it activates after every chopper passes through the trigger
.This is how my soundtrack code looks like since i have multiple songs

class CfgMusic
{
tracks[]={life,fail,panic,half,drop,fallout,finale,romeo,t ension,tribal};
class life
{
name = "life";
sound[] = {"\music\life.ogg", db+0, 1.0};
};
class fail
{
name = "fail";
sound[] = {"\music\fail.ogg", db+0, 1.0};
};
class panic
{
name = "panic";
sound[] = {"\music\panic.ogg", db+0, 1.0};
};
class drop
{
name = "drop";
sound[] = {"\music\drop.ogg", db+0, 1.0};
};
class fallout
{
name = "fallout";
sound[] = {"\music\fallout.ogg", db+0, 1.0};
};
class finale
{
name = "finale";
sound[] = {"\music\finale.ogg", db+0, 1.0};
};
class romeo
{
name = "romeo";
sound[] = {"\music\romeo.ogg", db+0, 1.0};
};
class tension
{
name = "tension";
sound[] = {"\music\tension.ogg", db+0, 1.0};
};
class tribal
{
name = "tribal";
sound[] = {"\music\tribal.ogg", db+0, 1.0};
};
class half
{
name = "half";
sound[] = {"\music\half.ogg", db+0, 1.0};
};
};