It sounds like a failed glue joint if you used hide glue you may be able to rub glue into the joint and wet the soundboard a bit either side so the board will expand  and 'clamp' the joint, some dowels pushed through holes in the bottom may be needed to help align the edges.  Of course a strip of parchment or cloth glued along the underside would be ideal but would involve cutting a hole in the bottom boards. If the crack is a permanent open gap then shimming would be appropriate. 

Richard

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