I have problems with the semper fi missions:

- audio problems (soldiers are shouting like idiots even if they are not present)
- mission bugs (missing enemies and soldiers, shooting/ floating weapons in the air)

I've seen that some of the posters have the same issues. (No matter what - you try to restart the checkpoint or even the whole mission, or you go back to the previous mission - nothing changes.)
I'm not sure if it works, but maybe I've found a solution on a Steam Forum:

If the player is using the numeric keys for anything in Crysis 2 the Semper Fi mission goes crazy. Reset the key bindings to default and change the settings again as you like, but leave the numeric keys.
Haven't tried it yet but it still sounds better than reinstalling the game or anything like that.

...

Okay! Tried it (couldn't wait.)
IT'S WORKING!
Anyone who can find this know that you musn't use the Numeric keys!
Thanks for the guy on Steam! He saved a lot of asses today...

I'm glad it worked....

Didn't work for you?
It solved even my audio problems in one step. Have no clue why.
I took off everything that was bound to any of the numeric keys.
Nothing?