The reason Snow leopards cannot roar is because of the way their throat is made.
The Snow Leopards Throats are made differently than a Lions throat so they do not have the ability to roar like Lions or Tigers can.
In short the Snow Leopard cannot roar because of the the physiology of their throat, and instead make a non-aggressive puffing sound called a 'chuff'
The Snow Leopard make sounds that are similar to those sounds that are made by other large cats, including a purr, mew, hiss, growl, moan, and yowl.
If the Snow Leopard had the same type of throat mechanism that would allow it to roar like other large cats then it would be able to roar.
But since the throat is made different in Snow Leopards than it is in other cats they cannot roar.