Physical Button Master Control is an Xposed module (Need root access) that allows users to use the music player ((play/next/previous/Volume Up & Down functions) when the device screen is locked. If you're a non-root user, you can try this module with the help VirtulXposed which said to be compatible with some of the Xposed modules without root access. This app creates virtual space to use Xposed modules. You can apply the Physical Button Master Control modules on the desired player after adding it to the Virtual space via the VirtualXposed "Add" function.