There are at least two artists using the name You Know Who:
1. You Know Who (also written YouKnowWho) is a Danish dance act consisting of Christina J. Undhjem, Martin M. Larsson, and Lars H. Jensen. They recorded some hits like "Guantanemera", "Good And Evil" and the album "You Know Who".
2. A punk rock trio based in the Southern California desert, fronted by Mike Pygmie, also of Mondo Generator.