Most any round can be suppressed to a level that the only noises you hear is the striker click, whizzing sound of the bullet in flight and, finally, the bullet striking the target, the last of which being by far the loudest. Semi autos can be very, very quiet too, providing that the action opening is delayed enough for the chamber pressure to drop so that the expanding gases from the breech are at an inaudible level, leaving just the sound of action working.
The key is to have sufficient suppressor chamber volume and gas flow characteristics to contain the high-pressure gases and release them through the muzzle opening in a controller manner. This is highly round-dependent, some produce higher pressure and more gas volume than others. Handloads with drastically reduced amounts of fast powder are the way to go.
As far as supersonic rounds are concerned, the flight noise, supersonic crack, is inevitable, but reducing the muzzle report makes it much harder to determine the direction where the shot came from by listening alone. Soundwaves travel perpendicularly from the flight path and direction of the bullet so the shot may well sound like it's fired from a completely different direction than it actually is.
That's just about it in a nutshell. The majority of my guns are suppressed, one way or another, and more often shot suppressed than not, regardless of the velocity of the ammo.