Volume adjustment slider trouble - iTunes 12.2.1.16

Hi,

Over the years of using iTunes, I've added around 11,000 songs into iTunes. I used to use the volume adjustment slide (right click - get info - options) to increase the volume of my music. I stopped using this feature some time ago.

Now, when I play music on any device synced with the music, the volume is always different as some of my library is '100%' and some is 'none' on the volume adjustment slider. I was wandering if there is a way to reset all of these levels for all of my songs to 'none' on the slider?

I have tried sound check but this doesn't seem to work too well, especially on older devices (e.g. iPod nano 3rd generation). I have also tried selecting all songs in my library and dragging the slider to none. This doesn't work either. I have tried dragging the slider from 100% to none on just one song at a time. I press 'Ok'. Then to check, I clicked get info on the song and looked at the slider. It had gone back to 100%!

This applies to iTunes 12.2.1.16 for Mac, running OS X Yosemite 10.10.4.

Thank you!