Perhaps not all other guns were silenced.
Also, if you are near the path of a bullet - even from a silenced gun you hear a loud whizzzzz!
Echos in the plaza confounded everyone, no matter where the shots, in part.
People can often judge where a shot came from by where and how it hit the victim rather than sound in and of itself.
As professional assassins usually make sure their guns don't emit visible smoke and there was a puff of smoke near the fence, this may have been part of the original plan...shots from 'Oswald' window and ONE other conspirator behind fence without silenced gun.
A shot from the Dal Tex building would be indistinguishable from one coming from TSBD and more liley IMO for the first shot.
I don't believe anyone heard a "whizzzz" from the Grasssy Knoll. People heard gunshots. As you say, maybe a shot from the Knoll was part of the original plan.
I do believe a shot from Dal-Tex is likely. It is just obvious to me that the conspirators weren't too careful in regards to the Knoll. Witnesses saw smoke and heard shots coming from there. If they were going to pin it on Oswald and the TSBD, why take this chance and not disguise the Grassy Knoll shot a little better?