Media volume switching to zero when screen turns off (Samsung S6)

[AC Question]

I've had my S6 for roughly 3 months, works a marvel, however when i lock my phone or the screen timer goes, the volume goes back down to zero no matter what volume its at. You can turn up the volume when the screen is black but when you turn it on and then off its goes back down to zero. Please help, music listening is becoming a hassle.