The way it's done is YouTube will post an advertisement for the song by the video, making it an ad for the music. When I posted my first video, not even THINKING about copyright, YouTube explained it to me. Some songs are available under a YouTube copyright license, and some don't even show up for some reason.
YouTube WILL contact you if there is any issue, and will mute the video if there is a copyright problem.
If the owner of the copyright doesn't like the attribution and advertisement under current YouTube policy, they will contact me through YouTube, and I'l take it down and re-do it.