Put this in your Description.ext where ambience is the name of your .ogg file.
Then call the file from a trigger or a script.
class CfgEnvSounds
{
sounds[]={ambience};
class Ambience
{
name="Ambience";
sound[]={"Ambience.ogg",db-20,0,1};
soundNight[]={"Ambience.ogg",db-40,0,1};
};
};