Not sure about the sound, but everything I've read says that pellets aren't designed for supersonic speeds. Accuracy drops off if the pellet goes supersonic. Use heavier pellets to keep the speed down. I suspect the speed ratings are exagerated anyway.
BTW- I have a .22 Marlin rifle shooting CB shorts that is a lot quieter than my RWS model 48 .22 side lever. The bullet/pellet speeds are supposed the be about the same, except the .22 bullet is a lot heavier than the pellet.