Do airguns get quieter the longer you have them?

No they don't.

some pellets make more noise than others. It sounds louder or quieter depending where you shoot it and the on the outside air pressure but other than that, the fundamentals of what makes air guns louder or quieter remains the same. 

It could be as simple as a different batch of pellets that fit th bore better, or more likely, your perception changed. 

Have you checked your tune lately?

Maybe you're losing your hearing from airgun overexposure :^)

There is a possibility that the mechanical sounds of the action could reduce because the rifle breaks in the more you use it. I don't think that the muzzle report becomes quieter though.

,Niels

If all looks good then enjoy the noise reduction

I guess it would have to do with the tune the type of pellet you're shooting and if your regulator is still working properly