AIMP player -whiel playing playlist songs go "dull"

[BarryBurgs]

I love the functionality of AIMP.  I have my PC connnected to a soundbar through a USB audio converter  (a Douk U2 Pro) via an optical cable.  The sound occassionally goes dull for a few songs (mp3 I converted from my cds some years ago) and then if I hit stop and play again reverts to a clear sound.  If I just let it play a few songs later it will revert as well.   I cannot pick any characteristics of the mp3s that cause it.  This does not happen if using Amazon Music or Spotify.   Any suggestions?

[Artem]

Do you use volume normalization or replay gain?