Video Asylum

Having purchased my DVD player, I now find myself going to the ole Blockbuster and renting DVD's - even when I have seen the VCR version, just to get the 5.1 effect. However, on the back of the boxes, they have what it has been re-mastered to. On the ones that say 5.1, no problem. However, on the ones that do not say 5.1, are they (5.1)? In other words, do some of these DVD's come in DVD format but not necessarly 5.1?