mp3gain 會修改檔案，但不會改到音頻資料，也不會解碼/重編碼
加入的 tag 是原本的音量資訊，讓你還原用的，跟 mp3 player 支不支援 replay gain 無關

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FAQ:
http://mp3gain.sourceforge.net/faq.php

Q: Does normalizing the mp3 degrade its quality?
A: No. MP3Gain does not decode and re-encode the mp3 to change its volume. You can change the volume as many times as you want, and the mp3 will sound just as good (or just as bad!) as it did before you started.