Presumably cockpit sounds and pilot communications are only clearly differentiated if the pilots are wearing headsets. I understand that pilots don't routinely wear headsets all the time in the cruise. Is this right?
I generally wear a noise cancelling headset while in cruise, especially when flying internationally. Many of my colleagues take off the headset and switch to speaker above FL 180.
I'll defer to the sea lawyers here, but I think the requirement to wear headsets below FL180 on most 'modern' airliners does indeed come from CVR regulations.