Just go into the level editor and click the sound icon on the side. Then load a ambiance sound and place it anywhere on the map. You might have to change the radius on it. Or you can go into the script and add
void OnStart()
{
PlayMusic(string& asMusicFile, bool abLoop, float afVolume, float afFadeTime, int alPrio, bool abResume);
}
asMusicFile - the music to play + extension .ogg
abLoop - determines whether a music track should loop
afVolume - volume of the music
afFadeTime - time in seconds until music reaches full volume
alPrio - priority of the music. Note that only the music with the highest priority can be heard! 0 - highest, 1 - lower, etc