Hi everybody,
I am learning AppleScript and this forum is a treasure trove.  Thanks so much!
My first project is this, I’m opening four video streams in QuickTime Player and placing the windows in a 2x2 grid.  This is the relevant AppleScript, it works great:
tell application "QuickTime Player" to open URL channel1
tell application "QuickTime Player"
	set bounds of front window to {0, 23, 720, 460}
	
end tell
tell application "QuickTime Player" to open URL channel2
tell application "QuickTime Player"
	set bounds of front window to {720, 23, 1440, 460}
	
end tell
tell application "QuickTime Player" to open URL channel3
tell application "QuickTime Player"
	set bounds of front window to {0, 460, 720, 900}
	
end tell
tell application "QuickTime Player" to open URL channel4
tell application "QuickTime Player"
	set bounds of front window to {720, 460, 1440, 900}
	
end tell
What I am stumped on is the AUDIO MUTING part. Ideally I would like 3 of the 4 windows to be muted so only the audio of one window is heard.
I tried invoking the “OPTION-DOWN ARROW” keyboard shortcut, which works when pressed manually inside QuickTime Player. But it doesn’t work via this AppleScript, or with many other variations I tried.
tell application "System Events" to tell process "QuickTime Player"
	
	repeat with w in (get every window)
		tell window "QuickTime Player"
			key code 125 using option down
		end tell
	end repeat
	
end tell
I also tried some variations of
  set audio volume to "0"
which I thought would be a cleaner way of getting there.
But I couldn’t get that working, either.
I’m sure I’m missing something basic, but I’m new to this and still learning. So thanks for any help you can provide.