如何在Mac OS X上将立体声输出缩混为单声道

我尝试修改中的设置com.apple.universalaccess.plist，但无法使其生效：

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

切换首选项的AppleScript：

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"