@Millie
Here's a gee whiz kinda video I made. It's a 9mm AR, with and without a suppressor. The magazine is loaded with three different kinds of ammo, in a repeated rotation.
1st shot: 115gr
2nd shot: 124gr
3rd shot: 147gr
And that repeats over and over. Only the 147gr is subsonic. You won't hear a huge difference without the suppressor (the first two sets of shots), but with the suppressor it's a drastic difference. You can also see how the subsonic burns with streaks more than a big blast with the unsuppressed shots.
As B00ger mentioned, there are three noises. The subsonic ammo eliminates one of them. The suppressor greatly reduces another. The majority of the sound you hear with the suppressed 147gr shots are just the action of the firearm itself...which would be similar to what you'd hear when pulling the charging handle and letting go on an AR, racking a slide on a semi-auto pistol or manipulating the action on a pump shotgun, etc.
iPhone mics and YouTube audio is definitely not real-world, but it's close enough that video is a good representation as to how much difference ammo can make in the perceived noise.
At a distance, subsonic sounds very different because there's no sonic crack wave with the projectile as it flies.