AC3 decoder question

Hi everybody, I just bought 5.1 speakers and tried some hd films with ac3 sound in KMPlayer using the internal audio decoder and the actors voices came from central but also from front (l+r) speakers, and it's the same with ffdshow decoder, but when I use AC3 filter, voices came only from central speaker. Also there are differences in these decoders in volume of voices or some effect like shooting. I never heared surround sound before and don't know how it's supposed to sound, but I can tell that every decoder sounds different. Also from rear speakers I hear nothing most of the time actually there is a very low sounds, when I put my ear close to speaker I hear it. So the question is what should I hear from which speaker when playing films, and which sound decoder shoud I use for hd films?