Hmm Am is a preposition and I believe that it still is here because it has caused a soft mutation on gwrando which a verb normally wouldn't do. I don't quite agree with it being translated as want because for me it gives more a feeling of "I am going to" but again I would say that is probably a way of saying it in welsh that can't be translated literally.
That's how this construction was taught in a class I attended. I was told it implied that "I am definitely going to do this and no one will stop me". In English I think we'd just do it with stress, so:
I'm sorry but I am going to list to this -- as opposed to
I'm sorry but I'm going to listen to this.