How to change Audio from Stereo to Mono on Galaxy S4

Wow, perfect! This solved my problem! Works on Galaxy S3 Neo as well, and also creates mono bluetooth output! I've built a bluetooth mobile speaker box with a built-in amplifier and battery. I'm only using one channel of the stereo amp. I was already looking at some other solutions for mixing stereo into mono using passive resistors before the amp, but it's much easier to solve the problem at an even earlier phase, from software! :) (The bluetooth amp doesn't have aux input and I can't get to the signal before the amp, it's all integrated on a board...)

Thanks...but there is a bug in S4...even if you use stereo headphones, this switch will change the music mode to mono.

@SaurabhJain2 It's supposed to. It's for when you use 2 ear headphones, but only are using one at the time, like if at work so you can listen to music, but don't want ear-buds in both ears. Need to turn on and off, no way for the phone to know what kind of headphone is plugged in unless it's through the micro USB port.