https://blog.faceutil.com/2017/03/23/mp3-%eb%b3%bc%eb%a5%a8%ec%86%8c%eb%a6%ac%ec%a1%b0%ec%a0%88-mp3gain-%eb%ac%b4%eb%a3%8c%eb%8b%a4%ec%9a%b4%ec%82%ac%ec%9a%a9%eb%b2%95-%e2%85%a4/