http://www.washingtonpost.com/news/post-nation/wp/2017/10/02/a-fire-alarm-from-gun-smoke-led-police-to-the-las-vegas-shooters-room-retired-officer-says]
Regardless of what the sound pressure at street level was, with a silencer the noise still would have been 120dB plus to the near by rooms which is still quite loud.
Here, the shooter was first located because all the gun smoke set off the fire alarm. Yes, he would have eventually been located based on the noise but even without any noise, he would have still been found and found first because of the smoke.
Had the shooter been using a silencer, more of the smoke would have remained in the room because the way a silencer functions is to reduce the sudden rush of gases from the muzzle. That results in gases coming out where the bolt is as well as the gasses going forward with less force. Basically it is like letting air out of a balloon slowly instead of popping it.
Using a silencer would have sent off the fire alarm much sooner. Not only that, the silencers get hot, quite hot. (Think about the muffler on your lawn mower.) Silencers heat up by 7 degrees per shot. After one magazine the silencer would have been glowing red hot and on the verge of failure. With the next magazine, the silencer would have failed (exploded) and disabled the weapon and possibly the shooter. There are lots of youtube videos of silencers being shot until they fail.
If he were to have put down the gun with a glowing red silencer, he likely would have caught the room on fire, which would have increased the speed at which the smoke alarm went off and caused him to stop shooting to put out the fire or flee the room.