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
An AppleScript that toggles the preference:
Yes, there's an option to enable mono conversion under Universal Access.
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.
Under iOS 7.1 on the iPhone and iPad, stereo to mono conversion can be enabled here:
(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:
Save it as an app and pop it into your dock for one-click monofying bliss!