I’m stuck on this - pressing play greys out the audio button in the editor and getting no sound.
To confirm, mute audio isn’t clicked on the game view window. Stopping the game, the button is switched off.
Have checked between a few versions of 2020 and 2021 and getting the same error. Also creating new projects …
The buttons are the same for me, and my audio works.
The mute audio button on the game tab is the playmode toggle.
The scene editor audio toggle seems to be used outside of playmode to preview placed audiosources around your audio listener. I’ve never used it until now, as I dynamically create all my audiosources, but I tested it now, and that’s how it works.
Does previewing sounds work?
If it doesn’t, check your windows mixer
You can also try to go to your project audio settings and reset them. Apparently that’s worked for a lot of people.
The sceneview audio button gets disabled because when you run your game you’re in the game view. Check your game view, it has a “Mute audio” button. Maybe you pressed that one? Again, the audio preview of the sceneview is automatically disabled when you enter playmode, otherwise you would hear everything twice.
Another source for issues could be the audio mixer of your OS. For example in the windows mixer you can mute individual applications.
in game mode it says stats and it says my audio lkevel -74.8 dB i think this might be the probrolem
I know that I’m late in the game for this one, but I just had this issue and fixed it. you need something that can hear the sound, just right click in the hierarchy and add a desalt camera and then you can click the grayed out audio button
I just had this issue, that I muted the sound in the Game Window because it was bothering me and then when I wanted to turn it on again the button was missing. I had to resize the game window to make it wider and it showed up 
Kinda stupid but Im leaving this comment, maybe will help someone 