Can't shoot in single or multiplayer!!
the problem occured by the 5th round of a bronze multiplayer campaign, started by losing the sound ingame so it wass all silence for about 2 or 3 rounds then the sound decided to return, but without ...
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I'm not sure of the cause, but your profile may be messed up and your process looks like it wouldn't have cleaned it up.
Try removing your Mass Effect 3 directory from your Documents folder (Just rename it).
Then launch a Mass Effect 3, it will detect it missing and restore defaults. All of your single player data will be gone (Unless you have cloud enabled, then you still might not have reset it, maybe disable cloud before starting Mass Effect 3)
Try creating a new game and progress from there until you can (try) to shoot.
If successfully copy saves and stuff back over, or revert to your original folder and try to fix it.
That is the nuclear option.
A quicker and more prudent, once Mass Effect 3 has launched, go to:
Extras -> Options -> Key Binding -> Reset to Default
