One side of headset / headphone does not work in Ubuntu 18.04.1

I recommend trying pavucontrol click the lock icon and confirm that both channels are up. You may have to move the affected channel up and down to fix the issue.

My Sony headphones displayed a similar behavior. When connected they worked fine, but once the computer went into auto lock, I would only get the left channel when I resumed play (some users report only the right channel).

I have seen some very complicated answers, and am sure that many of them are worthy of investigation, BUT PLEASE TRY THIS FIRST.

• When you have only one channel playing in the headphones, go to Settings → Sound → Output:

  

• Your headphones should be automatically listed in the Output Device drop box.

• Your Configuration should show High Fidelity Playback (A2DP Sink). I think that if this is set to Headset/Head Unit (HSP/HFP), the result is poor quality or mono sound.

• Make sure that the Balance setting is in the central position.

On Ubuntu 20.04, once I changed the balance back to the center position, the problem stopped occurring.

I hope this is of help to anyone in a similar situation.

Please correct me if my assumptions are wrong, but these are the available facts:

1. Headphone jack doesn't provide output on one channel
2. Speaker also doesn't provide output on the same channel
3. All the tweaking hasn't resolved the issue.

If all these things are correct, you might be having a hardware fault.
So, the question is, how do we verify if it's a hardware fault or not?

First, see if there is a HDMI output on your laptop. Connect to a TV using the the HDMI cable.

Next, start playing a movie (or anything with audio), and route the audio to HDMI. (use pavucontrol to do so; set HDMI out as default, and be sure that the stream (when being played) is routed to the HDMI output). Also ensure that that TV is unmuted.

If the audio plays correctly on that, then there is a high chance that this is a hardware issue.

Also check using one of these devices: USB Soundcards. These are cheap and a hardware fault would be easy to diagnose that way.

tl;dr: It might be a hardware fault; not a software issue.

I had the same problem. I opened Alsa Mixer and slid the balance all the way to the side that wasn't working. It started working. Then I moved it back to the middle and both channels are working fine again.