Are soundboard cracks a problem?
No! As long as the supporting ribs are still glued to the soundboard, and there are no buzzes or rattles, cracks are not a problem. Buzzes or rattles due to soundboard cracks can be eliminated by having the appropriate repairs done. There is a 9′ concert grand piano at our local University that has a soundboard crack open up every winter when the humidity decreases and closes again in the spring when the humidity increases. This situation does not seem to affect tone or tuning stability. Nor are there any buzzes or rattles!