So far I hav’nt found any Option or Way to turn on the Sound for all Videos.
Better would be, that the Page remember the last Setting for Sound of a Video (Volume, Mute on/off) or an Option in Settings - Local.
In the Settings I would call it
Mute Videos
Mute all videos by default.
It would be active by default until you turn it off.
Maybe even with an 25, 50, 75, 100% Volume Option, if the Page can’t remember the last Setting.