This is an A Capella version of Cara Mia, the "turret opera" from late in Portal 2. I don't really know why you'd sing this, but vocalizing music as beeps and boops is a big part of A Capella, so it works just fine.

If you want to see the original, you can watch it here. It's a spoiler though, so only watch if you don't mind seeing something from late in the game.