[Android] changing sound per ear?

Low-tech solution: try pulling the plug very slightly out, I had some headphones that would sound low in one ear if it was plugged all the way, but if I pulled it a millimeter or so it would sound normal.

As far as I know, Android has no standard audio balance settings. Certain phones might have it due to manufacturer customizations (I think some Samsung phones may have it), but otherwise you'll need to use some third-party mixer which might require that you're using xposed or that your phone is rooted.

The first thing you should do is rule out a hardware problem with the headphones. Plug them in any other device and check, whether they behave the same way.