Background music stopped working??

[AC Question]

Hi. I've been using Android for years and I usually download a background music player to play music and run other apps and even play videos simultaneously (on Mx Player). I don't know what happened exactly but it doesn't work anymore? When I play a video the music pauses and when I play the music the video pauses. I've reinstalled both apps and it's still doing the same. Does anyone know what could cause this to happen?