Elena Siegman is the singer for the hidden songs you can find in certain maps in the Nazi Zombies game mode. She works with Kevin Sherwood who does the instrumental work to make these songs. Her songs are used in Treyarch's games (Call of Duty Black Ops , World at War) for the Zombies series.
Her page is here.
Her facebook page is here.
The songs below are listed in order they appear in the maps. All are sung by Elena.