The number of discrete channel signals present in the audio source will determine what will come out of your speakers (without help from you audio drivers) . MP3 files will be (mostly) in stereo. So in order to produce sound in 4 (or more) speakers, your audio driver is going to have to synthesize the additional channels. This is sometimes referred to as "fill" for the rear channels. I'm not familiar with your sound card, but most Realtek motherboard drivers have a check box for this feature.
For movies (or other audio sources) that contain the discrete 5.1 or 7.1 audio signals, your drivers should detect that and send the appropriate signal to the appropriate channel.