That's a good thought. Initially the density is pretty normal and I'm sure there's a loud sound, but once pressure drops the air wouldn't transmit as much sound

That's a good thought. Initially the density is pretty normal and I'm sure there's a loud sound, but once pressure drops the air wouldn't transmit as much sound. I don't see enough detail to really know how high they fly once the depressurization happens. I'm guessing that, if possible, they descend to an altitude where the supplementary oxygen is not critical for preserving life so the loudness could go back up. I couldn't find if there's a "typically safe" maximum altitude that is used in depressurization or it's situational. Probably someone with better flightradar knowledge could figure out the altitude with time.