This is the ever popular sound problem. Two possilble solutions.
1. Check out
this post for a step by step fix. If you have noticed distorted dialogue/sound fx this will fix that too. This is the 'best practice' fix.
2. You can just ram these guys until they are destroyed. They will not attack you and the mission will carry on. As far as i am aware the sound problem wont cause you any problems later on (which makes me pretty sus about the whole thing, a more cynical person would say microsoft left this in to bugger you up if ur using DivX - the manual clearly says you
may experience problems with certain third party sound drivers).