At sea level, the speed of sound is roughly 1127 feet per second. When a moving object, such as a bullet, breaks that threshold, there is an audible shock wave that sounds like the crack of a bullwhip. Most of the noise inherent with shooting a rifle, however, comes not from the bullet breaking the sound barrier, but from expanding gases rapidly escaping the rifle’s bore. It’s the latter bang that causes hearing damage, reveals shooters’ positions, scares game, and works Mrs. Walney down the road into a fit of agita. So while subsonic ammo does eliminate the downrange crack, and does reduce the bang at the muzzle somewhat due to its lower velocity, it won’t make your centerfire rifle sound like mouse feet on felt. It’s just one way to mitigate the noise.