The good thing about the awesome dor yeshorim rap that came to light last week is that there’s no music, just beatboxing, so you can listen to it during sefirah.
The problem is that it’s still kol isha. I’m not referring to the singing, that’s obviously kol isha, but I think even the beatboxing is; I don’t know, someone should show this to a rov and ask a shailah. The reason I’m less worried about the singing is because rap may not be considered music according to most poiskim. I know when I was in yeshiva we paskened that goyishe music doesn’t have a din music so you’re allowed to listen to it during sefirah.