That's odd, mind you I have two identical MAM weapons that make a noise when the button is pressed and one has stopped working, there's no way of opening it up to replace the battery..sob
How about filling out a Hasbro customer support ticket over at hasbro.com, they may be able to help via an email back.
Between here and saving the universe again! "That's one small step for a MAM, one giant leap for MAMkind"