In European Spanish, "Z" is always pronounced as "TH" in words like "think" or "thanks", never like words such as "then" or "though". The phonetic symbol for this sound is θ. Examples are: Caza Corazón Zumo. Now, there is a rule that says that "Z" may never be use before "E" or "I". That means that words that should be declined using the combinations "ze" or "zi" must change the consonant. For this purpose you have to replace it with "C". Lápiz, in plural should be "lapizes", but this rules does not allow it, so it becomes "lápices" and "C" is pronounced as if it was "Z" (θ). So, the combination of all vowels in Spanish for this sound should be: za ce ci zo zu θa θe θi θo θu "S" has different sounds in different regions of Spain, it sometimes sounds close to a whistle other times it sounds almost like a "sh" and other times it sounds as "s" in English in words like "son", (never like "s" in "vision" or "cousin"). This is the same sound of the "S" that is most used in American Spanish. I hope this helps you.