doubt it glass wouldn't suppress noise to any discernible degree
plus, the guy was in the same building, so a lot of the sound would travel through the structure, itself (beams, flooring, etc)
it's clear that there were at least 3 or 4 shooters, as you can hear audibly different firing signatures--some sharp and very loud (close), some a little "muffled" (farther away), and some sound like they are way off in the distance (long range support shooter(s))
i wouldn't be surprised if this guy filming turned out to be in on the arms deal since he was so calm