Why is my audio playing app stopping the audio when my phone screen turns off?

[ResortTOHope]

I have a TracFone LG Ultimate 2(an LGL41C) It has android version 4.4.2. Won't let me update. It no longer has a phone plan associated with it. Has an micro sd slot so I want to make it into just a player using some kind of file reading media playing app (suggestions?). Every every time the phone screen times out, the app stops playing the audio. Worked with settings, but the only way the app keeps running, is if I keep the screen on. I turned the lock screen off. Do I have to upgrade to get a setting that will solve this?

 

[B. Diddy]

Welcome to Android Central! Any power-saving modes turned on? Or 3rd party battery saver or "RAM Booster" apps installed?