Mekomos Hamegulim is only Assur when the intent is to have Hanaa. Singing is Assur even without explicit intent.
The Gemara in Kedushin is hard to base things on, anyhow. As the conversation went on you see that he didn’t want to send regards to his wife even through the husband. It seems like, as MDG said, Kol Isha is sort of a relative, or borrowed, term.