Our SRT has suppressed 5.56 and .308 weapons. The 5.56's are AR's, and we have two M/O teams with the .308. So I hear them at least 4 times a year at Weapons Quals. Going back to when I could run with the herd, I carried a suppressed H&K MP5N, and then a suppressed M4. I might not be an expert, but I definitely know what they sound like. That being said, by far the quietest was the MP5N. The action cycling made as much noise as the shot. The M4 makes a short, quick "whistle pop", and has a supersonic round, which is a noise that cannot be suppressed. Same goes with the .308; it still makes a pretty loud pop when you fire it. And this is at an outdoor range. Indoors, I could zero in on any of those guns (except maybe the H&K) fairly quickly. The real problem would be the echos bouncing off of walls. Indoors, you are taught to look for dust from the muzzle blast when narrowing down a shooters location.
I've never fired a suppressed pistol, but I would guess that a suppressed .45 is quiet, but still audible a good distance away. Forget about what you see in the movies. Only a suppressed .22 is that quiet. Well, that and an MP5N.
There are posters here who have suppressed weapons. They know what a suppressor will, and won't do.
All that being said, my guess would be that the chaos in the immediate vicinity of an active shooter...suppressor or not...would eliminate any "cherry picking" after the first shots. Humans are sight-hunters. They react quicker to what they see than to any other stimulus.
One of the "rules" of ASR is run towards the people that are running, then when you don't see anyone running, slow down and start your sweep. Deliberate movement when you don't hear shots; dynamic movement when you do.
I wasn't there, but it sounds to me like the VBPD folks who responded took the fight to the bad guy. Good for them. Takes balls; big ones.
And yet another reason for those who can legally carried concealed to do so.