It's greyed out. As shown here, unlike the mic icon.
However now I noticed if I swipe left/right to another camera and then back to the troubled one the icon shows up and I can turn the sound on.
Definitely a bug. I still have no luck turning it on when trying to playback clips.
I am have audio issues where the sound icon is greyed out on my cameras with audio during live view and playback. I know they are recording audio because the saved clips have the audio icon and playback is fine on the PC. I have one camera in a group and if I view it in that group the audio icon...