I perfectly know what you're talking about because you've plenty of those in english... for your luck, we haven't much of those ambiguous letters in italian. One is 's' as you rightly pointed out. Another common one is "n" as in 'pane' and 'valanga'. in the first word "pane"(bread) you should use the sound you make with 'punish'. While with 'valanga' you should make the sound at the end of each gerund verb form like "going" (the sound of -ing which is written as /ŋ/ in the International Phonetic Alphabet). other allophones are: 'c' --> Casa, Cile. 'g' --> Gola, Giove. Maybe there are even other ones but right now I can't tell. If you need to practice italian we could chat on skype cause I need to improve my pronunciation. add me as a friend if you wish, and then you'll see my skype contact. don't forget to mention this italki message in your authorization request though. best wishes, daniele.