Android per-application volume changer

On my Android 4 phone I use the music app "Deezer" to ear music... Now I would like to simultaneously use PDF Voz (or other text-to-speech apps) to listen to some texts while I ear soft music in background. While both simultaneously play, my current problem is that both output to the same "music stream" so I'm unable to independently control the two volumes: turn down the volume of the music and turn up the volume of the voice. I need a solution to *independently* change the volume of Deezer and of PDF Voz.