If you're having issues with voice chat try lowering the slider for silence. Mine was set at 99% , once I lowered it my mic was able to work without screaming into it :D
There's more to it than that. I've tried different settings, but the key thing is that sometimes it works and sometimes it just doesn't, it's totally intermittent. In some missions I'll see my voice indicator when I press the button, and in some I don't.
What I suspect might be going on, but I have no proof, is that if all the other players have VOIP turned off, it just doesn't bother, and I see no indicator. But that's a really confusing UX. They could put VOIP indicators on all the players near their health bars or something.