The speed of sound is 1,126 ft/s.
Any ammo that leaves the firearm at velocities less than 1,126 ft/s is considered sub-sonic. Most of the report you hear when firing a weapon is the effect of the sonic boom created as the bullet breaks the "sound barrier" - or - accellerates to a speed faster than 1,126 ft/s. Keep that from happening and the muzzle report will be much quieter.
The ft/s muzzle energy rating on the box is an average of a manufacturers chrono tests out of the specific barrel length they choose for testing. That being said, it could range from a 6" pistol barrel to a 24" or longer rifle barrel. Since energy increases the longer the gasses expand in the barrel, longer barrels will generate more muzzle energy than shorter barrels up to the point that gas expansion ceases. So, the muzzle energy on the box is not always what you get out of your weapon. Long story short, ammo rated at super-sonic velocities (greater than 1,126 ft/s) out of a 22" barrel will likely produce sub-sonic velociteis from a 16" barrel. Most .22lr ammo will be sub-sonic when fired from a short barreled handgun.
As far as accuracy goes, the idea is that the sonic boom created when the bullet passes the speed of sound affects the bullet's trajectory thereby affecting the point of impact on target. Since every single bullet and powder load is minutely different, this added variable is undesirable when shooting for pinpoint accuracy. Remove the sonic boom from the equation and in theory the result should be an increase in consistency on target. Personally, I have noticed a significant increase in consistency on target using subsonic ammo when shooting with or without a suppressor.
Hope that answered your question...