My craftsman riding mower doesn't seem to cut evenly and sometimes leaves portions untouched??

Check your belts and make sure they are tight,make sure your blades are getting up to speed,you might even check you engine rpms

There is no mistaking the sound of a Craftman tractor when you engage the blades. Does it sound like it always did? I have several Sears tractor and they all sound like a helicopter taking off when the blades engage. When you say uneven are you talking left to right one edge higher than the other or whole patches width of the deck left uncut?