Your sternum is the bone that runs down the center of your chest between your pecs. Protected by minimal muscle and being so central, it can easily become inured and is affected by a range of different movements. If you have a painful sternum when you sneeze, or if it cracks, then this may cause you concern and/or discomfort. Fortunately though it is normally nothing, and is usually a result of one of the following issues:
A hairline fracture or just a bruise on your sternum itself can be the cause of pain in the center of your chest. If you have recently received a blow to this area then this may have either bruised it, caused a hairline fracture or chipped off some of the bone. In this case it may be painful to touch as well. It is recommended you see a doctor to ascertain whether there is a break though usually there will be little they can do for it (they may use a bandage to apply some pressure and support to the area). Make sure to get plenty of rest and to avoid the bench press for a while.
The muscles surrounding the sternum (the inner pecs) can sometimes be torn or pulled and this can cause pain when you sneeze due to the physical jolt this causes. This may be a result of going a little heavy on the bench press or during a netball pass, and again rest is recommended to give the area time to recover.
Another good reason to visit a doctor (who will likely refer you to a physiotherapist) is that you may have inflammation in the joint (the costosternal joint where the ribs meet the sternum). When you sneeze or stretch this can cause a sudden pressure change in the area as air escapes and this is what causes the 'popping' or 'cracking' noise that can occur. This trapped air can also just be a result of the shape of the joint and may not be due to any inflammation.