Now, you said the aliens had no music of their own. So this would be their first introduction to music? Now, when humans are introduced to music for the first time, what do they like? I would say classical, in the case of infants, but the aliens would not be infants. They would be potentially less likely to be scared or upset by loud music. So, they would be completely intelligant being that could judge music from a completely objective point of view.
Classical seems very likely, but part of me wants to say they'd like hip-hop. Or dance music. Something with a repeatative groove. It would hypnotize them, like a lava lamp. And that is how we will defeat our alien invaders.
