Greasy Fork is available in English.
Pause or mute HTML5 audio/video on leaving a tab and restore them back on returning.
By default, this script will pause instead of mute. To make it to mute instead of pause, change the muteInsteadOfPause variable in the script from false to true.
This script will only pause/mute if the current tab has been fully loaded before switching to other tab. IOTW, it won't pause/mute if the current tab is still loading when switching to other tab.