When using customised sound controls, sometimes the little icon showing what I am controlling does not appear.
It shows the slider on the right instead of the one on the left. One way I found to get the icon back is to switch Media Output and back, e.g. to "This phone" and back.
Note that I altered the image to show my experience because I cannot reliably reproduce it to capture the actual screen, but it looks exactly like the slides on the right.