There is a recording here of Hatikva (The Hope), as sung by Jewish survivors at Bergen Belsen on April 20 1945. It is part of a report filed by Patrick Gordon Walker for the BBC. Hatikva is now the national anthem of Israel. (Via Michael Greenspan and Judith Apter Klinghoffer.)
Update: The link is broken but try here. The folks at Israel Reporter had their site hijacked but have made a video now using the mp3 of the survivors singing.