The default state is “unmuted”. The action behind it all is .setMuted() so it should be “true” to mute and “false” to unmute. This comes from the player. Muting is not the same as decreasing the volume to “0”. You can still adjust the volume on a muted file, but you won’t hear anything until you unmute.
What I find a little odd is that the exact same action is published differently depending on its position in the title 😉