rsgain (really simple gain) is a ReplayGain 2.0 tagging utility for
Windows, macOS, and Linux. rsgain applies loudness metadata tags
to your files, while leaving the audio stream untouched. A
ReplayGain-compatible player will dynamically adjust the volume of
your tagged files during playback.

rsgain is designed with a "batteries included" philosophy, allowing
a user to scan their entire music library without requiring external
scripts or other tools. It aims to strike the perfect balance between
power and simplicity by providing multiple user interfaces.