Apologies for commenting on a post in 2021 but somenot all screen locks will offer you an Media Control option. I am working on a solution myself and the closest is to install VLC Player and use their widget for Media Controls by placing it on your Home Screen. Be very careful for some reason VLC Player likes to buffer as if it's being throttled by a lot just go to YouTube and check the share button as VLC Player and you should be able to play music in the background even when your Smartphone falls asleep. I haven't so far found any other solution besides change your theme on your phone to one that offers media controls on your device. If I some how get VLC Player to stop buffering it's so far the best alternative instead of having to pay for YouTube scam services.