The mantras and each of their letters have a precise way of being pronounced, but if you are an Italian it is much easier for you because the way of writing actually corresponds to the way of pronouncing most of the times, always taking into consideration that the K it is guttural, the T and the D are like in the English TH and DH, the C is sometimes sweet as in the Cen variation of the sixth Rune, other times it is vibrated in the throat like a cat's hiss (almost all times it is the second case to be the right one); etc.
It depends on what language you speak. For example, the Klim mantra is pronounced exactly: KKK guttural LLL EEEE MMMM (this is how an Englishman would write the pronunciation) However, an Italian would actually write the pronunciation as: guttural K, L, I, M. So apart from the guttural K, nothing changes in the writing of the pronunciation. But, apart from this, the pronunciation of the mantras is precise and should not be changed to one's liking.