now i know this thread is more than 5 years old but I have a definitive solution for anyone who uses windows 7 and realtek audio manager.
in order to get the left and right channels playing in one ear u have to go to the speakers tab in realtek audio manager and set it to any of the settings that say cd quality or dvd quality (any of the settings that have 16bit) then click ok you'll have
to do that each time u change it other wise nothing will change.
this will work but the trade off is the reduction in audio quality. my recommendation just go up the settings list in the default format tab until u find a setting that gives u mono and the audio quality u want.
to test it afterwards just use realteks speaker config tab with the play button to test if it plays both sides
