Hey,
thank you for your answer. I selected "Suspend Audio Engine When Cakewalk Is Not in Focus." Now at least its possible to listen to music without closing the Catwalk programm ^^. But i want to play and record instruments while listening to some .mp3 files at the same time. When Catwalk is not in Focus, theres no audio output from Catwalk (that means i cannot hear the instrument through my USB Headphones) and recording isn´t working.