Contrasting Design Conventions

Working on a website recently that incorporates a music soundtrack I noticed that audio and video controls function in opposite ways.

The video play control shows you what will happen if you click on the button. So, if a video is playing, the pause button will show. And if the video is paused or stopped, the play button displays.
However, with audio the reverse is true. If a website uses music or sound effects the on/off control will show you whether sound is currently on or off, not what will happen if you click it.
It must be because I’m so familiar with how these controls function that this intuitively feels right to me. However, on reflection, it’s odd that they’ve come to work so differently.
The more I think about it the more it seems strange that the video play button displays the opposite of what it is doing. I can’t think of another instance in which a button works in the same way.