It looks like this unmuted: https://discussions.apple.com/content/attachment/e909bffc-81...
and this muted: https://discussions.apple.com/content/attachment/e3e9c230-e4...
I believe the reason it always has the slash is to distinguish it from the "change volume" button (which, when pressed, pops up a volume slider). But it's still confusing.
As someone who studied two years of cognitive sciences as part of my degree, this is not necessarily true. Lack of information can often lead to even more cognitive load as the user then has to imagine virtual scenarios, an cognitive process which is linear. Visual processing however is largely parallel, faster and less consuming, as we have evolved to live in a visually complex and noisy environment.
Of course, admittedly if all you have is a play button and a mute button (or some other very simple player) it probably doesn't matter much.
I think Twitch's UI is a good example of where the clutter goes too far. It's loaded with tons of distracting bits and bobs. Makes sense for their business model (most of these relate to how you can spend money on the platform), but from a user perspective it's unpleasant.
But a mute indicator isn't telling you how other people appear to you. (That would be "deafen".) It's telling you how you appear to other people, which you have no other way of knowing.
I also can't live in a world where play/pause is opposite to the way it is now: play icon should mean the media is paused, pause icon should mean the media is playing, as you suggest.
If we constrain ourselves to only having two buttons (one play/pause, one mute/unmute), and only differentiating them by the shown icon, then I'm afraid it just has to go with the convention. They are inconsistent with each other, but intuitiveness beats consistency I guess.
> The answer in my opinion is to separate the concern of the action and the state and distinctly show both at the same time.
I don't think this is practical, especially on mobile. There is a need for these UIs to be concise, and even if there is enough space, each visual element comes at a cognitive premium to the user.