i think it is something like puting the audio source on a non code affected object
like your code plays sound
but if its on the same object this mean it will cut that sound to play the other one
so maybe you should make 4 empty objects with your code and apply your code to each one of them so your code only plays only one sound from each object and when it tries to cut an audio it will not find any
you can research it more i think you will find better explaining but that is my way to fade the audio