Bring the soda to a simmer in a medium heavy saucepan. Add the dried oranges and simmer and steep until the orange rinds soften, about 5 minutes.
Mix in the cranberries, sugar, pomegranate molasses and cassis. Bring the mixture to a boil over medium-high heat and cook, stirring occasionally, until the cranberries burst, 10 to 12 minutes. Using a potato masher, lightly mash the cranberries, leaving some whole so that the mixture resembles chunky jam or preserves. Transfer to a bowl and refrigerate until cold, about 4 hours (the sauce will thicken as it cools).
Can be prepared 4 days ahead. Cover and keep refrigerated.