Balancing earphone sound
Android itself has a.f.a.i.k. no balance setting (which is a shame!). 

But you can try in Googles Appstore "Bass Booster and Equalizer" - which installs itself as MusicPlayer and allows - amongst many other audio settings - to adjust the balance.

The only disadvantage is that the settings are limited to the application itself (the music player), and not system-wide effective.