What player are you talking about (Player, ExoPlayer, TaifunPlayer, …)?
Background means here, of course, that the app is NOT closed, because background services (tasks) will not be available for Kodular, AI2 etc. in the near future.
The problem with stopping / stuttering the sound when streaming in standby (sleep mode) has already been discussed extensively in the Kodular and AI2 forum.
Btw, the Player component and the TaifunPlayer extension work without any issues in the background (unless it is streaming).