When you select the background music and make sure that it's an overlay and not the one to be selected. Because once you select the main audio it will cancel the background audio. So, When selecting the main audio make sure that your background audio is overlay
I have been trying to test the app but it does not get beyond showing me a screen showing all the apps saying "this permission allows an app to show things on top ..." when I select I cannot advance from there? I am using a Samsung S9 android phone. Can u help..