I have a Hornet and a .221FB. The Hornet is a little quieter; that may be partially due to having a heavier, longer barrel than the CZ sporter, but I'd hazard a guess that the .22 Hornet is the quietest over-the-counter centerfire cartridge you can buy. No way is the .223 quieter.
If I were a gun/ammo maker, I'd be looking at quieter, short-range c-fires as the wave of the future; we're simply running out of land and it's getting harder to shoot at longer ranges. That, and/or making suppressors easier to get in the U.S., as they are in the U.K., (they may even be mandatory in some areas there.) I looked at internally-suppressed like those at johnsguns.com, but seem pretty expensive.