LG zone doesn't allow me to listen to music with screen off.

Hi, everyone, i just bought a LG Zone phone a couple of days ago, i am noticing that when you turn the screen off, the phone disable all the apps, including the ones that should stay in the background, like music, i've had other smartphones and this is the first and only one with this kind of problem.

Do you know if there's anything i can do to listen to music with the screen off? is there any app i could use to force my music player to stay in the background when the screen is off?

I have rooted my phone already, so root permissions wouldn't be a problem, the model of the phone is LG-x180g LG Zone, android 5.1 Lollipop.

Thanks in advance...