I have a feeling this is always the case - maybe someone can correct me - but I believe it is probably working fine even though the mute has a line through it - it looks as though the button only changes it's shade - as if pressed in when muted, but normal colour when not muted (i.e. the same picture though).

It probably doesn't get confusing if the microphone works, but it definitely confused me when testing my microphone! Maybe we can push for getting the actual picture to change - i.e. cross out when muted, no cross when it's active.

Unless of course I am talking rubbish, but can't test it because of my microphone!