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 command line 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!