To burp, you would have to be conscious in order to let the air out of the stomach. To sneeze, your nose would have to be bothered by a stimulus which would probably wake you up before it happens. Hiccups are believed to happen when the body needs extra oxygen, so the body would probably not need a sudden extra amount of oxygen when you're asleep anyway.
oops hit the submit button before i finished lol!
My son's both sneeze in their sleep and don't wake up - they are 9yrs and 4yrs old. #
Also my DH is quite capable of belching in his sleep (and its opposite number!) without waking up but I think that has more to do with him being pished at the time hahaha.
You can burp, sneeze and hiccup while you're asleep!!! I stay up all night and I know what people are like when they're asleep! It's funny also listening to what people say when they're asleep!