I want to play mp3 audio within app . file is to be save in asset. Is there any permission need from user to play audio. ? mp3 file is like a song. It can be play in background.
No, You don’t need to ask for any permissions from the user
You must try before asking. Just use Sound component and set Source to the file you want to play it doesn’t require any permission
You can’t do anything in your app while in background, once the app closed its activity stops until the user opens it again.
But there’s a paid extension for background tasks; BackgroundTasks Extension
Mark as solution if it helps 
There are many misconceptions about the term “background”. Background as mostly used means only that the app is not visible (e.g. after pressing the home button or after time-out (in idle / sleep mode), but not closed. “Background tasks / services” means that an app can also execute somthing if the app is closed. That’s is not possible with AI2 & its clones (Kodular, Thunkable, …) without an extension.
remove “phone call” component to avoid this permission query.
It shouldn’t ask for that permission since it only needs to play audio files from its assets. If it’s your app remove anything related to calls in your components, if it’s not yours you mustn’t use it because it gets from you personnal informations that it doesn’t need