𝐌𝐮𝐟𝐟𝐥𝐞𝐫

If you've even been in a loud environment, and put on your headphone, you may have noticed the sound is dampened quite a bit.
A similar process happens with the muffler. The very loud noises that comes from the engine are canceled out and reduced by reflecting off the walls of several chambers in the muffler. The sound that is then left exits the muffler and is released. Mufflers are great at reducing sound and the engine would produce very loud noises without it.
Make sure you have a legal muffler so you don't get pulled over. Some people want to use less restrictive mufflers to help modify their cars to pull in more oxygen and perform better. If you hear loud engine noises from your car, get your muffler checked.