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?

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

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"

Yes, there's an option to enable mono conversion under Universal Access.

System Preferences > Universal Access > Hearing > Play Stereo as Mono.

Found the answer here.

EDIT 2014-05-25: As explained in Apple's documentation, the stereo to mono downmix option can be found under Accessabilty on Mac OS X 10.9 Mavericks.

System Preferences > Accessibility > Hearing > Audio > Play stereo audio as mono

Under iOS 7.1 on the iPhone and iPad, stereo to mono conversion can be enabled here:

Settings > General > Accessibility > Hearing > Mono Audio

(Forgive me for digging up an old post...)

On Mac Os X Yosemite (maybe on Mavericks too), the Accessibility window is slightly different and the Applescript posted above doesn't work. Here's the same one with a slight modification:

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 window 1
end tell
quit application "System Preferences"

Save it as an app and pop it into your dock for one-click monofying bliss!