Yes, the sound on the various videos could be edited. It's not conclusive. But it is -- if the evidence wasn't tampered with -- strong support for the multiple-shooters idea, and counter-evidence for the use of AR-15s with bump fire stocks (exclusively, anyway).
I honestly don't have a good idea why someone would want to cover up the multiple-shooters angle. Well, I could speculate and come up with several possibilities; the problem is that I don't have evidence for any of them. What I do have is evidence that the official story doesn't explain, and in many cases, actually contradicts the official story. And, increasingly, thanks to SpyGate, I have evidence that the government and the FBI are not worthy of blind trust.