insert a video/audio into document, then select the object, media playing toolbar will appears and there is a mute button.

First time when I click the play button, I cannot hear anything, then I see the mute button, and click it, but nothing changed....and there still a red cross icon on the small horn. I click several times, but cannot see any change. So, I thought the function doesn't work...

I spend some time and found I also muted my computer...

Then I tried again and found the unfriendly usage:

with unmute state, hover mouse over the button, it's highlighted. Then I click, it's also highlighted, with the original icon remains... I cannot see any change, so I thought it doesn't work. 

After I move away the mouse, I found when it's mute, the button is highlighted. otherwise it's not. But the highlight state changing is not so easy to be seen.
If the icon also changes, it will be better.