Sam2, I’m not questioning the heter, it makes sense. I’ll bl”n check out the B”Y. Cherrybim keeps on quoting R’ Moshe on it, and not providing a source, and oddly, he now cited three teshuvos, none of which say this heter.
As far as slow music, I actually questioned R’ Belsky on this, because he’s against fast acapella music because it brings simcha and is associated with “rikudim um’cholos”. So I figured, mah nafshach, if so, then slow music, even instrumental, should be okay (and someone whom I respect paskened this way). He didn’t agree, but didn’t really explain.