I found out of that, if I set the
const unsigned long pauseTime = 6000; // how long between PIR detections
to 7000 instead of 6000, then it actual works. The LED then blinks for 1 second more than the mp3 plays(the mp3 file is 6 seconds long). So it must be something to do with it sees that the dfplayer isnt finished, maybe due to some milliseconds delay to power the dfplayer off, and then it continues in a wierd loop.
But I think I can use it with 7000 instead.