There are many misconceptions about the term “background”. Background means only that the app is not not visible (eg after pressing the home button or the app is in sleep mode, black screen), but is not closed.
So if the screen goes into sleep mode or you press the home button, the sound (ExoPlayer) should continue to play …
Background task means that the app can execute somthing even when it is closed, this is not possible with Kodular and the other builders (Ai2, Thunkable etc.). So when the app is closed the sound will stop.
So if the sound stops even though the app was not closed, you’re making a mistake somewhere.