Ear Plugs Achievement
Kill an enemy while your rifle fire is masked by a loud soundYou will probably get this naturally if you are using sound masking (curved lines in the top right corner of the screen indicating a loud noise to mask your rifle fire), but this can easily be unlocked during Mission 2, 'Mittelwerk Facility'.
At the very start of the mission, directly in front of you there will be an enemy at the top of a watch tower. Line him up and wait for the speakers to start. A sound masking icon will appear in the top right-hand corner, so when it does, shoot the soldier and this achievement will unlock.
If for some reason it doesn't, just reload the checkpoint. This is almost story related as there will be many more opportunities throughout the campaign to unlock this.