For the most part, yeah, but I think for "special" songs, they bring in other people to sing for them.
But then again, I'm not too sure either, I don't believe a lot of the "real voice" things on YouTube have been re-pitched properly; whenever I've done them, it comes out Ross using his natural (the voice he uses for Dave) when speaking for Alvin, then he speaks in a lower tone for Simon, whereas everyone else's repitches make it sound like he's pitching his voice higher for Alvin, while speaking in his natural voice for Simon.
You can tell I've been bored, lol!