Hello Vetroni, There is of course orthography in Italian or any other language. Maybe you expressed your question in an inadequate way. This very particular change in the pronunciation of the consonant "C" when you put "amico" into its plural "amici "is part of the rules of Italian orthography. The spelling in Italian language is quiet consistent in general ,even in the case of heterographics ( which are the letters that could be spelled in more then one way ,like the "c" in your example) ,you can find them listed and learn them or just practice them in your course of readings. You have to regard also some other peculiarities among them the spelling of digraphs ( ch for example) and trigraphs (gli for example) .Buena Fortuna :)

It was JAcket not "Jachet". "Gia" sounds like in JAcket.

A good way to remember how to solve this particular issue (the sound of C and other letters) is remembering that the C sounds like K only if followed by A, O and U. The sound of the G sounds like in the English "get" or "garden" in the same cases. When G and C are followed by the vowels I and E the sounds change. With the E the G is pronunced like in "gErman", with the I (always pronounced like EE of "ChEEse" in Italian) and like the English name JImmy with the I. The same with the C. C with the vowel I always sounds like CHEEr. C with the E always sounds like in CHEst. In order to give a hard sound to the syllables "Gi" and "Ge", and "Ci" and "Ce" you have to put an H between the G and C and the vowels. In order to get sweet or soft(in Italian we say sweet) the sound of the syllables CA CO CU and GA GO GU you have tu put the letter I between consonants and vowels. "cia" always sounds like in "CHAt" "cio" always sounds like in "CHOp" "ciu" always sounds like in "CHOOse" "gia" always sounds like in "JAchet" "gio" always sounds like in "JOke" "giu" always sounds like in "JUne" Check this out: http://www.articlesbase.com/education-articles/pronouncing-italian-vowels-and-consonants-709974.html