mono audio stopped working

Hey!

My university lectures for some reason have an audio track only for one earphone. I wanted to change the sound to come out of both of my earphones so I researched how to do that. The first thing I learned to do is to go to audio MIDI setup section of system preferences and change the channels so that instead of hearing the track from left ear I heard it in my right ear as that is my better ear. Later I learned about the mono audio setting, and after turning it on I was happy to have the sound in both ears. Then I went to turn the MIDI setup settings to how they originally were. However after I did that, the mono audio stopped working. I have turned it on and off and restarted my computer and put the MIDI settings back to the way they were and mono sound still won't work.

Any ideas on how to fix this?