With songs written by slaves as cole said, there was little to be found online, this is what I could find:

Sweet Chariot Spirituals
http://ctl.du.edu/spirituals/freedom/civil.cfm



SPIRITUALS AS GOD’S REVELATION TO THE AFRICAN SLAVE IN AMERICA
http://people.bu.edu/wwildman/WeirdWildWeb/courses/theo1/projects/2004_woodbine_onaje_and_clay_darryl_and_jennett_pauline.pdf


Songs of Freedom
http://www.osblackhistory.com/songs.php


Surprisingly for me there was however a lot of songs about slaves written by abolitionists including this list of civil war related songs:

Library of Congress Civil War Sheet Music
http://lcweb2.loc.gov/diglib/ihas/search?view=thumbnail&query=slave&submit=GO&sort=titlesort&hiddenquery=%2BmemberOf%3AcivilWar&view=thumbnail&field=subject