Use of a, an

It is generally believed that words beginning with 'an' should be used before words begininning with vowels a,e,i,o,u. But we cannot use an before words 'University', 'one rupee note'
This needs some discussion. It appears that first letter of word is not important. The sound is important. 'One' is pronounced as 'vun' and 'university' as 'Yoon-i-var-siti'. Hence use 'a' before these. On the other hand, we use 'an' before 'hour' and 'honour' as 'h' is silent.