I've had a suppressor for my AR for a little over 7 years. The suppressor is an AWP Optima specifically made for a .223 though I can (and do) shoot .22 Rimfire through it (via a .22LR conversion)
The sound when shooting either a .223 or .22LR High Velocity is, as far as my ears can tell, approxitmatly the same. What you hear is not the muzzle crack (expanding powder burn) so much as the sonic crack from the bullet going down range.
With .22 Subsonics you hear the hammer hit, bot cycle and the bullet impact the target (if the target is close enough).
A friend has an AWC Anphibian (integral suppressed Ruger Mark II). This weapon "bleed" the velocity of the bullet to subsonic levels and is extreamly quiet for every shot.
Like SDC stated... the object of a suppressor is not to silence the gun (not possible) but to slow the gasses caused by the powder burn as they leave the muzzle (think of an "in line" care muffler). This does a couple things...
1 - Changes the sound of the muzzle blast so that it is "omni - directional" (ie you may hear the weapon (if the suppressor is ineffective) but you will be unable to pin point where the shot came from.
2 - Reduces the Db level of the muzzle blast to levels where it is hard to hear out past 25 yards (depending on caliber) or so (an effective suppressor). With my Optima if you stand back, off to the side approx 20 yards you hear the sonic crack and then a "Hissssss" as the gasses leave the suppressor.
3 - (Additional Benifit rarely mentioned) - Eliminate any muzzle flash. Usually, with my Optima, the first shot (.223) out of the suppressor will produce a light blue flame approx .75" long and .22 caliber as there is enough oxygen in the suppressor so that the powder burns completly through the suppressor. Subsequent shots produce no flash as there is now no oxygen in the suppressor .
A lot of mention is focused on shooting subsonic ammo and that it is more desirable then shooting hyper-sonic ammo. You have to remember that with an effective suppressor the sound of the muzzle blast will be greatly reduced (see above) and that the sonic crack you hear from a bullet travels perpendicular to the bullets path and that to hear the sonic crack the bullet has to pass by you (or by something that will "reflect" the crack).
What this means is that if you are down range and a bullet passes you that has been fired from a suppressed weapon you will hear the crack but not the weapon fire. If the sonic crack of the bullet sounds like it came from the right or left of you then, in fact the weapon was fired either from the front or back of you (and conversly, if the crack sounds like it came from the front/back the weapon was fried from the left/right).
I've had a friend shoot my AR while I was standing down range (75 yards) off to the side (approx 25 yards). Disclaimer - Do not attempted this at home. Skilled professionals were used for this experiment and no animals were harmed. I started facing him and closed my eyes... as he shot (1 shot every 3-4 seconds) I turned towards the sound I heard till I was facing it. I ended up looking towards the bullets path with him to the right of me. At 75 yards I could not hear the gun fire at all, only the sonic crack as the bullet passed me.