Yeah, too many people getting caught up in Macross 7 thinking, when this show isn't Macross 7. Using terms like "spiritia" and "sound energy".
If I had to guess, I would say they are effected by emotional response. When Ranka screamed in the cockpit with Alto, they reacted and bugged out. This wasn't singing. If singing brings Ranka and Sheryl a great amount of joy, perhaps this attracts them. I dunno.
In each series, music has been used differently. In SDF Macross, it was culture shock. In Plus it was hypnotic. In 7, it was, well, magical. In Zero it was, I dunno, a controlling force? Anyway, you get my point.