Some piano owners have been dismayed to learn from their piano technician that their piano has a crack in the soundboard. While this may sound disconcerting, it does not automatically mean that the piano has to be rebuilt or replaced. The fact of the matter is that pianos are sensitive to changes in the climate and in the room conditions. The greatest factor affecting piano tuning and stability is humidity. Over many years, solid (usually sitka spruce) soundboards dry out. This drying out process eventually could lead to a split occurring along the grain somewhere on the soundboard.
The criteria as to whether it is necessary to repair the soundboard is whether there is a distortion in the sound of the piano at various frequencies or notes. In other words, there might be a distinct "buzz" that develops when certain notes are played. This would mean that a repair should be done. In most other instances, there is no sound distortion so no action needs to be immediately taken to repair the soundboard.
A basic repair for this in an upright piano is to insert glue between the soundboard and the ribs closest to the crack and draw the soundboard back to the ribs either by screws or by some other method. This usually will eliminate the offending noise when the piano is played.
In grand pianos, this condition is a little more serious mainly because of the visibility of the soundboard. Soundboards in grand pianos are very visible especially when the top lid is propped in the open position. Depending on where the crack is, the piano technician will usually have to loosen the tension on the strings directly over the crack to access the affected soundboard area. Then he will take a specially shaped chisel and widen the crack so that a wooden shim can be inserted into the widened area. This shim is glued in and then, after the glue has dried, the shim is sanded flush to the existing soundboard area. A light finish such as lacquer is applied to make the repair look good and the strings are then replaced.
In severe cases, the entire string set and plate have to be removed to expose the crack. This is a shop repair which means that there will be an additional expense in moving the piano to the shop and returning it to the customer's home. It is important to get a professional estimate done with a reputable technician before this repair is undertaken.