Why are you making this so complicated? The simplest solution is just a button with text on it. But sure, maybe you want a symbol instead, they do look nice and take less space.
So when sound is playing, the button should have a symbol that conveys "mute", like a speaker in a stop symbol. If you hover over it, the tool tip should say "Mute" or something to that effect.
If the button is clicked, then the sound is muted. Now you could change to a symbol that conveys "unmute" like just a speaker symbol. Or you could let go of the whole button concept here because it's pretty clumsy and ambiguous, and just use a toggle switch instead. Could just have the text "Sound on" - "Sound off" on either side, or symbols if you prefer that.
You should never assume that the user knows anything. The UI should convey everything the user needs to know, trying to use some sort of convention doesn't work because there is no convention. Avoid using signal colors because as you alluded to that also gets complicated.
So when sound is playing, the button should have a symbol that conveys "mute", like a speaker in a stop symbol. If you hover over it, the tool tip should say "Mute" or something to that effect.
If the button is clicked, then the sound is muted. Now you could change to a symbol that conveys "unmute" like just a speaker symbol. Or you could let go of the whole button concept here because it's pretty clumsy and ambiguous, and just use a toggle switch instead. Could just have the text "Sound on" - "Sound off" on either side, or symbols if you prefer that.
You should never assume that the user knows anything. The UI should convey everything the user needs to know, trying to use some sort of convention doesn't work because there is no convention. Avoid using signal colors because as you alluded to that also gets complicated.