Sins only has a certain selection of tracks loaded at any one time, which cover a broad base of situations (combat, quite, winning, losing etc.). It won't start playing a new song unless the game determines the atmosphere has changed enough to create a new category, or until it finishes the old track, in which it will play another track it thinks is suitable for the atmosphere. If you have very long tracks like I do it may indeed seem that it doesn't change much (keep in mind the default music is only 1-2 minutes per track).
Otherwise, look in the SoundMusic.sounddata file, and mess with the numbers there because you must not have enough music for all atmospheres, causing it to play only what it has.