This article from the Creative site might help you out.
In a nutshell, if you have your speakers connected via the analog ports (3.5mm jacks) on the sound card, then you should be able to adjust the software to play 2-channel sources through all speakers.
If you have it connected via the digital connections (optical or coax), then you will not be able to listen to 2-channel sources through all speakers without a separate decoder.
Okay so please bare with me as I know very little about sound cards, sound speakers and anything of the sort. I know what 5.1, 7.1, and 2.1 and that stuff is, but otherwise please explain to me as if you're explaining math to a 3rd grader! So i got a set of the Logitech X530's, they are 5.1 surround sound, and i love them! The only problem is that they don't play music out of all the speakers all the time. When i play music from iTunes, they play out of all the speakers, but when i play you tube, or anything else like games, or videos they only come out of the 2 main front speakers. I have looked up every solution I know, to no avail. I have configured them, bought a new sound card(it supports 5.1) and i have gone into the advanced settings and set the speakers to 5.1 settings. I don't know if this will help but in my playback devices under my speakers and over to this tab called "Sound blaster" it has its own advanced setting and under that is where i set it to 5.1, but there's these 2 other things I am not too sure about called EAX effect(which i do not have enabled) and CMSS-3D (which is enabled). Would these have anything to do with it?? Thank you in advance for taking the time to read this and for helping me out guys! I appreciate it!