That is an uncommon issue as far as I know. It sounds like your OS is outputting in Mono. OK, please load up pulse audio volume control.
Click on the Output Devices tab. Make sure Port is set to Speakers. Also, make sure that it says its outputting Stereo. If it is, then lets check balance.
On the right side, there are 3-buttons just above the Port selection box. The middle button is a channel lock button, make sure the channels are unlocked.
Doing this will release the ability to manually adjust the left and right channels. Make sure that both channels are balanced in the middle.
Once your sure they are balanced, lock the channels again to prevent them from changing. If you are still having issues with this, we will have to help you digging deeper.
But this is what I know, to help you now.