Windows 7: Altec Lansing Speaker Issue

I have Altec Lansing speakers model VS3251 (sub, center, left, right, rear left, rear right). When I test my speakers under "Speaker Setup" on Windows 7, each of the speakers works perfectly and accordingly. However, if I go to any program other than "Speaker Setup", my rear left and rear right speakers do not work. I realize that the sound file must be 5.1 encoded. I downloaded many 5.1 sound files to test and none of them worked. What am I doing wrong here? Thanks for any help! It is greatly appreciated!