KaraokeRG is a simple wrapper around the mp3gain program that allows you to easily apply ReplayGain adjustments to your mp3+g zip collection.
You tell it the directory under which your karaoke files are stored, start it, and it will recursively go through and extract each zip file, run mp3gain against the mp3 file, create a new zip, and if all of that went properly, it will replace the existing zip with the modified one.
Builds are currently available for Windows 64bit, Fedora/RHEL and derivatives, and for Ubuntu and derivatives.
Note that this software depends upon the mp3gain program already being installed on your system on Linux based builds. For Windows, the mp3gain binary is included in the installer.
Linux packages are provided in the same repos as the OpenKJ program. The package is named karaokerg
Windows installers can be found via the downloads link at the top of this page.
For more info on what ReplayGain is and about mp3gain, look here: http://wiki.hydrogenaud.io/index.php?title=ReplayGain