I did a test a while back with "Standard" Velocity .22 LR ammo - - most all of it was "Sub-sonic"... Figure anything below 1,100 fps is "sub-sonic", and most of the ammo I tested that was "standard velocity" was running around 1,080 fps...
As long as the bullet isn't breaking that 1,100 fps mark, the sound barrier won't be broken, the round won't de-stabilize when it passes from "super-sonic" to "sub-sonic", and the report is reduced considerably. When shooting the standard velocity ammo, I don't even bother wearing ear plugs - - the sound of the firing pin dropping at the squeeze of the trigger is as loud as the report of the round discharging.
For ammo... Aguila standard velocity, Fiocchi standard velocity, CCI standard velocity, Federal 711B's, Pistol match ammo, etc. all shoot well from my bolt-action long gun - - accurate and quiet.