Nothing will annoy the neighbors (and your wife) more than a cracked or missing muffler on your lawn tractor. Luckily, replacing a Cub Cadet muffler is pretty easy for most models as they use a clamp instead of require a weld. Replacement mufflers are also pretty inexpensive, so we would recommend buying a new muffler even though a used one might be $5 less.
If your muffler is just cracked or loose, you can try replacing your muffler clamp or using some JB weld to fix it. If the crack is very large, you should go ahead and replace it as it is cheap and easy to do it yourself.
To extend the life of your muffler, you can buy some high temperature spray paint and give it a good coating to make it last a little longer.