It does not apply to gentiles, they are not bound by the mitzvot. Gentiles and Jewish men can listen to a woman singling if they don't see their image, and gentiles can listen to whatever they want.
The reason is no the men don't fantascize about her while she's singing, which is very hard not to do if you know what she looks like, even just admiring her beauty more. Gentiles so long as they aren't copulating with her are fine.