So to be clear, the right channel of the headphones comes on when the device is switched to mono, correct?
And this problem exists with any set of headphones plugged in, correct?
If both of the above statements are correct, is the problem occuring with any app that you play audio with?
Here are a couple of apps that I use all the time.
I would like for you to try them and tell me what, if any, difference you have with this issue.
First, they are apps that I use, so I can help you navigate them quite easily if you have any problems with them.
Secondly, they are pretty cool apps that function well.
Be sure to switch the device back to stereo mode first.
The first app is called RadioDroid, and it will allow you to listen to worldwide broadcasts of radio stations that also stream their feeds online.
Simply start the app, enter the call sign of your favorite radio station, and listen to the feed.
Take note of whether both channels are working.
https://f-droid.org/en/packages/net.programmierecke.radiodroid2/
The next app is a music player.
The link is for an old version, from before the app had ads.
It is called BlackPlayer, and has some cool features.
One of these features, the one that we are wanting to use for this experiment, is a volume balance control between the left and right channels.
Start the app, and give it a bit to find your audio files.
https://blackplayer-music-player.en.uptodown.com/android/download/345945
After it starts, go ahead and select some music to play.
Swipe to the right, and select:
Settings
Audio
Select Equalizer
BlackPlayer Equalizer
Now go back to the app's homepage, swipe right again, and select Equalizer.
Don't turn the switch 'on' yet. Leave that for later fun with this app.
For now, swipe left, and that will reveal a secondary effects page.
Here at the top are the left and right balance controls.
These are actually pre-amp volumes, which are separate from the main volume control that you are familiar with.
The default levels are 100%.
So, if only one channel is working, turn that channel all the way down.
Turn the main volume {the familiar volume control} up.
Is there any sound at all?
At the very bottom of this page in the app is a volume boost control.
CAUTIOUSLY adjust this upwards, listening for any increase in volume, anything at all.
I say cautiously emphatically for two main reasons-- the volume could suddenly come on at a very loud level, and also, if you hear any kind of scratchy sound or distorted souund at all- at any volume, bring the booster volume down immediately.
Please get back with me with your results.