This is a common difference between American and English/British pronunciation. For names we tend to use the European pronunciation, whereas generally Americans anglicise it.
I met someone called Gutman, and he introduced himself to me as Gut (rhyme with nut) man. I did not know what he meant until he wrote his name down. Then I said “Oh! Gut (rhyme with foot) man!”. Or, when my boss’s daughter got engaged to an American called Levin, which we would pronounce ‘Leveen’. When I asked him where the ‘Leveen’ family were from, he snapped and said “its LEVin!” (to rhyme with Kevin).
To answer LB’s question, I think you can’t force people to pronounce anything the way you want them to. Just say it your way often enough until they copy you.
Incidentally, JK Rowling (whose name rhymes with bowling). has said that she is the only person to pronounce the name Voldemort correctly. She intended the final ‘t’ to be silent, giving it a French twist. Now who calls him that, (if they are brave enough mention his name at all)?