I miss the old convention of showing a button with shadows indicating it's been pushed in vs. showing it with just a flat plane when it's not pushed in. A button that shows that it's been pushed in with a triangle icon means it's playing, clicking or touching it will stop it from playing. A button that doesn't indicate it's been pushed in with a triangle icon is not playing, clicking or touching it will start it playing. Bonus points if there is a small virtual led that turns on when it's pushed in.

Fewer and fewer devices we have access to these days have physical switches on them, though.