I agree with Mandy.
The distinction is between voiced consonants and vowels (b, v, m, etc.and vowles) and unvoiced consonants (p, f, k, etc.). So, kick -> kicksss, dog -> dogzzz, guy -> guyzzz.
When you have a word that ends in S, the plural is formed by adding "is" at the end, and that's pronounced like the verb "is", in "That is my car"... "iz", z-sound.
I would also like to add an interesting thing, while we are at it.
That rule (z after voiced consonants, s after unvoiced consonants) is a very good rule to keep in mind, but if you listen closely you will notice some (or many) native speakers don't seem to follow it in their speech. You will notice it's common to hear they use the s-sound (or a very similar sound) after a voiced consonant to form the plural, where a z-sound would be expected. If you notice that, then you are noticing a final devoicing. I notice that all the time. In other words, the z-sound is "devoiced" to some degree, or even completely, so it becomes very similar to an s-sound or even a pure s-sound.
Hey guyz! <-- expected z-sound after vowel.
Hey guys! <-- devoiced, it sounds like an s-sound.
I should also point out that devoicing doesn't change the preceding vowel length, but that's another story.
I don't mean to say learners should do it, I just mentioned final devoicing because it's a feature they'll definitely come across if they listen to a lot of English. I suggest sticking with the traditional rule, since there are much more chances you'll sound natural that way.
Oh, and by the way, welcome to Englishforums, Mandy.
I saw you are new... Hope you'll like it here.