The Spanish letter Z is pronounced like the soft C (the letter C in front of E and I); that is, it is pronounced like a TH (in Spain)* or an S (in Latin America).
*This is what you will hear in the sound files.
Note: The letter Z can never precede an E or an I in Spanish; it is replaced by the letter C. (Why is this?)
- lápiz -> lápices
- yo comienzo -> yo comencé
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.