I agree with what the guy before me has posted. The game detectives website: http://wiki.gamedetectives.net/index.php?title=Battlefield_1, is awesome and has gotten me through all the way to the 8th code. A lot of time is spent actually having the morse code translated and then the decoding to actually work. Keep using the exit the game and go back into it, ensuring that you are in a different server. I believe I read that if a set of headphones is picked up on that server, they will not show up again. Not sure if this is true, but after 2 runs along the maps from the website listed above, I exit out and go back into a new one. You'll get there and definitely download audicity, it will kind of come in handy.
Good luck.
