Pronunciation of the final "s", whether for plural noun forms (cats, dogs) or for third person singular verbs (he sleeps, walks), depends entirely on the preceding sound.

First say the word to yourself without the "s". How does the word end? Do you use your voice box or not?

To demonstrate;
try saying cat, walk, sleep, then isolating the last sound (t,k,p). You'll find that no sound comes out - these are 'silent' or 'unvoiced'

Now try saying wood, dog, rub and isolating the d,g,b. You'll be able to hear these sounds as they are 'voiced'. You can also 'feel' the vibration if you put your hand to your throat.

In fact, /t/ and /d/ are the same sound, only unvoiced and voiced.

Now try adding the final 's' to these words: cats, dogs etc - isn't it easier and more natural to say /cats/ but /dogz/?

The grammar rule is that you say /s/ after an unvoiced word ending, /z/ after a voiced one, but I don't really teach my students the rule - I just ask them to do what seems easiest and that seems to work.

An NB for completeness:

For words that end in a sound that makes it really difficult to simply add /s/ or /z/ - such as horse, watch, badge - you need to add /iz/ (never /is/)



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Note added at 4 hrs (2010-10-23 07:36:21 GMT)
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More NBs:

All of the above depends on [b]sounds NOT spellings[/b]

All vowel sounds are voiced.