The most worrisome crack caused by moisture is a crack that goes through a tone hole. Passing through a tone hole will result in a leaking, unpredictable instrument. These cracks should be taken care of as quickly as possible. L&P offers tone hole crack prevention, which almost entirely eliminates the possibility of tone hole cracks.read more.
If you notice a crack through a tone hole you don't need to panic! Just follow the advice from the article
Tone hole replacement
A crack could open up again, anytime. Therefore, it is important to replace the broken wooden tone hole with a crack-free synthetic material like Delrin as soon as possible. The tone hole-pad connection will stay good, even if the crack opens up again. You might get a leak between the wood and the tone hole insert, but this can easily be fixed, even by yourself.(see article above.
To replace a tone hole is very safe! We have replaced thousands of tone holes over the years and it never changes the sound or tuning response.
Don't get the instrument pinned under any circumstances!!! (Read also: Crack pinning )