loudgain is a versatile ReplayGain 2.0 loudness normalizer, based on the EBU R128/ITU BS.1770 standard (-18 LUFS) and supports FLAC/Ogg/MP2/MP3/MP4/M4A/ALAC/Opus audio files. It uses the well-known mp3gain commandline syntax but will never modify the actual audio data.
Just what you ever wanted: The best of mp3gain, ReplayGain 2.0 and Linux combined. Spread the word!
Youlean Loudness Meter is your ultimate free VST, VST3, AU and AAX plugin for loudness measurement.