If we are going to add a mic check, it should work on all browsers; if it doesn't work, it shouldn't show. When I click the button on Chrome, it moves to the right and a little grey bar appears to the left of it. However, the grey bar doesn't respond to my voice (I can successfully make a recording, so it's not my mic). I'm told it is supposed to look green and respond to voice (go up and down?).
On FF and Safari, the button doesn't even move to the right when clicked.
On IE, I have not seen this.
Instructors who think that a mic check is in place would feel more confident to just allow one attempt (no longer using 2nd attempt as defacto mic check). But if it actually doesn't work on all browsers a student might use, that could be a problem. So my preference is that it work on all current browsers (and FF24-26) or not be seen on any of them, with being visible on some but not others a distant third.