Thanks for the suggestion, I just ran some tests and this would be at least a workaround as it seems to play the normal notification sound while the media player is on pause. From what I read Sleeper is not able to completely stop the media player but only pause it right?

Does anyone know where the beep that gets played when the media player is running comes from? The quick and dirty solution would probably be to just replace the soundfile with a louder sound.