I'll just add to this, 9mm, or any handgun really, is still very loud. The reason it doesn't seem as loud is because when most people are shooting there are two main things happening.

1. They're behind the barrel, normally this doesn't matter much, but the sound is at least a little directional, so being in front of it is going to make it sound much louder because you're hearing the initial explosion, not an echo.
2. Most people aren't shooting it in their house, they're at a gun range. The space in front of you at the range allows for the sound to travel and the pressure to spread through the room, slightly reducing the impact of the sound. Shoot one in a tiny room and it's going to be much worse for you.

Again it's still really loud, but the context of where the sound is being made does make a difference. Obviously larger rounds will be louder, but that doesn't mean rounds like 9mm are safe for your ears at all.