My Magnum is on a raw diet, too. From my experience, he generally has harder stool when he's eating stuff with a lot of bone in it. (I have not, however, ever(!) been able to hear it as it hits the ground–:eek: ). Generally, this stool is very whitish in appearance and very crumbly after a day or so.
Here goes: What color is the stool? Does Nature's Variety chicken medallions have a lot of bone in it? If it does, I'd probably add some more meaty meat to it--raw chicken breast, for instance.
It's not necessarily a bad thing, but I, personally, don't like to see that type of stool, for more than a few days. To me, it means they're getting too much bone in their diet and not enough meaty meat. (I subscribe to the 80% meat, 10% organs, 10% bone way of raw feeding.)