macos - How to downmix stereo output to mono on Mac OS X

Let's just open and close this:

Is there a built in way to downmix all stereo output to mono on Mac OS X?

Answer

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I tried modifying the setting in com.apple.universalaccess.plist, but couldn't get it to take effect:

f=~/Library/Preferences/com.apple.universalaccess
sudo defaults write $f stereoAsMono -bool true
sudo chown $USER $f.plist

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An AppleScript that toggles the preference:

tell application "System Preferences"
    activate
    reveal anchor "Hearing" of pane id "com.apple.preference.universalaccess"
end tell
tell application "System Events" to tell process "System Preferences"
    click checkbox 2 of tab group 1 of window 1
end tell
quit application "System Preferences"