A post for those searching a similar issue…
For sometime now I have only had sound out of left channel of the headphone jack on my Mac Book Pro. I checked for lint in the jack (happened to my phone once). No lint. As it turns out, the balance on my sound output settings was set all the way to the left! A Google search confirms that others have experienced this, too. The balance will adjust left on its own. Strange, but an easy fix: Just slide it back center.
Find it here on the mac: System Preferences > Sound > Output