Mutt is spot on about there being a door or two between cockpit and cabin in the G550, which effectively blocks sound from the cabin.
I have had pax in a loud and boisterous card game in the cabin without anything being audible - all we had in the cockpit was the stale smell of Chinese tobacco products and that vile chemical scent of moutai.
Thank goodness the pax can't hear what we say in the cockpit, or I would be out of a job.
Happy days