Precision Volume by Stixel
Precision Volume solves the problem of having songs in your library with different volumes and prevents you from having to adjust the volume between each song.
Just open Precision Volume, hit play, and adjust each song to a comfortable volume using either the in app song shift slider or the external volume buttons on your device.
The next time you play that song, PRECISION VOLUME AUTOMATICALLY RECALLS THE SONG SHIFT YOU USED FOR THE SONG THE LAST TIME IT WAS PLAYED.
HOW IT WORKS:
The volume used for each song is simply calculated by adding the Song Shift for each song to the Master Volume, making it easy to adjust the volume across all songs for loud or quiet situations using the Master Volume, while also remembering differences in volume between each song using the Song Shift.
Developer Website: http://stixel.weebly.com/