You will only get the full 5.1 sound effect when you use a 5.1 encoded sound source. To test the effect, obtain a copy of a science fiction DVD (The Matrix or similar) in which full use is made of the capabilities of surround sound. Note that as this is for testing purposes, it doesn't really matter whether the film is to your taste or not. You need it to demonstrate the full effect of 5.1 sound and these films certainly do that!!
MP3 files are normally in two channel stereo (left & right) so you will only get the front L & R speakers working, although some speaker systems are capable of offering an emphasised bass response through the subwoofer.