Unlike a simple volume boost, ReplayGain analyzes the entire file and writes metadata that tells your player to turn the volume up or down without re-encoding the audio . This preserves quality.

Download and install Audacity from the official website (audacityteam.org). Also install the FFmpeg library when prompted — this allows MP4 import/export.

If you have any specific, legitimate use cases or questions in mind, I'm here to help.