There is no central authority that dictates how certain combinations of letters are pronounced (unlike French for example), and the same combination of letters can be pronounced in different ways. Take the homograph "bow" for example. Take a bow for tying a bow onto the bow of the ship. Sometimes spelling has little relation to pronunciation. How do you say Worcester source? Alnwick Castle?
The pronunciation of English words is established by common usage, not spelling, so a word that is not in common usage has no correct pronunciation.