Riding Lawn Mower Help?

I sold my mower to a friend and i go back down and calls me back up a hour later saying that the lawnmower is making a clunking noise. I don't know if this could cause it but the belt is dry and cracked. But also the it sounds like a gear in the transmission. It will go clunk and go again. The shifter to put in forward neutral or reverse moves back in fourth. The wheels when they spin in the air didn't seem like they were getting much power. Either way i put it in forwar or reverse it will making a clunking or hitting sound.