Your Mazda is an overhead valve engine so there are no tappets and the valves don't need to be adjusted. It does have variable valve timing though. An exhaust leak won't make you lose power, not noticeably anyway, and you should still be able to hear it by revving the engine by hand with your head under the hood. If it's wireless have someone else rev the motor. The car should be under warranty up to 80,000 so take it back to the dealer.