Until I saw the comments in this thread, I had no idea the briefing tapes weren't just the same as the spiel you get while in the helicopter heading to the mission. Played through the entire game without listening to 1 of them.
I had a number of really resigned groans when I discovered game options that are completely undocumented like 40+ hours in. I did at least know about loadouts though.
cthomer5000's comments