What can be done in a situation where left/right channel sounds louder then opposite channel? I know that songs' volume can be normalized, but can normalization also be performed on channel/s?
You can use Audacity to normalise channel volumes. I'm using an old copy of Audacity 1.3.13, which is a beta version, but just about any version should have these options - some plugins may have to be installed, but they're easy enough to find.
Note that this reduces the volume of the louder channel and increases the volume of the quieter channel.