I accidentally clicked the mute icon and I did not realized it at once because no audio was playing on this device. 

When I came back wondering why no audio is played I saw that the slider representing the output device is disabled. I first thought the hardware is somehow disabled. A bit later I found out to click the loudspeaker icon on the left side to unmute.

I did not see that earlier because the "normal" and "muted" loudspeaker icons are visually quite similar at the given size. The red "minus" symbol looks like black for me. I have a green/red vision impairment to some degree which means the smaller a red (or green) area becomes the more it looks like gray.

So, a textual representation of the muted state would be appreciated.