You can buy a suppressor that is permamently attatched to your airgun w/o any problems. Meaning it must be integral to the barrel. If it can come off and be modified to go on a firearm, and reduce sound by only 1 decibel, you must have the appropriate tax stamp to legally possess it.
Some airguns do benefit from a sound moderator. Spring airguns do not, the spring makes more noise than the air release. However, CO2 and PCP do. To illustrate this, simply pop a balloon. It makes a loud pop . Same principle as with CO2 and PCP.
Also, anytime you break the speed of sound (@1200fps) you get a loud crack. Most airguns shoot under 1100fps, so if you have a pcp or CO2 gun it can be quite quiet.
I know a lot of folks get all excited about high velocity airguns, but once you break the speed of sound, it defeats the purpose of an airgun. Not only that, but 98% of the pellets aren't designed for those speeds, and accuracy suffers.