A man’s ghostly voice speak-sings from the black screen: “Rock-a-bye baby, on the treetops …” It’s a tentative voice, unused to intoning lullabies, the voice of a man who was just released from prison. When he was convicted, his twin children were 45 days old. Now, they’re 21. This father’s voice is one of dozens collected in the ongoing documentary project “A Father’s Lullaby” by current MIT Open Documentary Lab Fellow Rashin Fahandej. It comprises a compilation of recorded lullabies and oral histories from incarcerated fathers separated from their young…