^Correct. Subsonic rounds are made to be used in conjunction with a suppressor. Since they are slower than the speed of sound they do not have that report when they fly, and because of the supressor, the initial explosion from the primer and the powder is quieted down so you don't hear much of anything besides the impact of the bullet.
Using a subsonic round by itself means you don't hear the bullet crack through the air, but you will still hear the shot being fired and what is hit by it. Everyone will hear it.
A traditional BB gun would work, I'd lean more towards using a pointed nosed pellet to take them out though. Also some of the newer, higher end pellet guns shoot as hard as .22s and they are legal to shoot in most populated areas, unlike a .22lr.
I have an old school .22 pellet rifle that shoots at about 650fps, subsonics fie at about 1,000fps. It will definitely take out some critters and is quiet.