>who gets paid if DJ so-n-so samples a cover song?

Both parties. The writer of the original version, and the perfomer of the version he sampled.

**EDIT**
The explanation is, because once somebody covers ( and releases ) it, it then becomes a NEW "original" recording. So DJ So-and-so is in fact sampling another artist. Think of how many versions of "My Favorite Things" or "Imagine" there are. If somebody samples say, Tracie Spencer's version of Imagine, She gets paid, AND Lennon gets paid.

>what if someone sampled a random person on youtube
>singing/playing a famous song?

You can't really copyright that ( assuming you're talking about just some cat singing into his laptop camera ).

But the original artist might come at him if he releases it. It might fall under interpolation, but I'm not sure. But the person who performed it on YouTube ain't got shit to say about it.