With Youtube could be an error because you will spend more 4G Data with the videos and when you close the youtube app or the mobile phone is blocked, music stop (you need to place contents in maximized window). If you have not a special music account as Amazon Music, Spotify or similars (to spend less data), you can use mp3 files with Samsung Player as you said, but you will need to download source files to a folder in your mobile phone to listen it or you can use a SD card to save music folders too.
When multimedia contents are inserted in your phone , you could need to restart terminal for update multimedia files to show list and files correctily.
