When you are teaching American English it is important to recognize the difference between voiced sounds and voiceless sounds.

A voiced sound is a sound that causes your vocal chords to vibrate. A voiceless sound does not cause your vocal chords to vibrate.

American English has 15 voiced consonant sounds and 9 voiceless consonant sounds. All of the vowel sounds are voiced.

Try this so you can see what I mean.

Put your fingers lightly on your throat and say zoo. You should feel a vibration when you say the /z/.Keep your fingers on your throat and say Sue. You should not feel a vibration when you say the /s/.

One reason why it is important to know the difference between voiced and voiceless sounds is because the letter ‘s’ at the end of a word can have a voiced sound /z/, /ez/ or a voiceless sound /s/.

Plurals, possessives and third person singular verbs all end in ‘s’. The ‘s’ can have three different sounds
/s/ a voiceless sound
/z/ a voiced sound
/ez/ a voiced sound

A voiceless sound follows a voiceless sound.

When a word ends in a voiceless consonant the added ‘s’ is pronounced /s/. Here are two examples:
sock/s/ -since the /k/ in the word socks is a voiceless sound, the ‘s’ ending is also voiceless
map/s/ - since the /p/ in the word maps is a voiceless sound, the ‘s’ ending is also voiceless.

If a word ends in a voiced sound the ‘s’ ending is also voiced.
Here are two examples:
love/z/-since the /v/ in the word loves is voiced, the ‘s’ ending is also voiced and sounds like /z/.
dad’/z/- since the /d/ in the word dad’s is voiced, the ‘s’ ending is also voiced and sounds like /z/.

The exception is that words that end in the three sibilant sounds take the /ez/ plural ending.
/∫/ pushes
/t∫/ watches
/dgz/ judges

This /ez/ or schwa + /z/ sound adds a syllable to the word.

It is important to recognize the difference between voiced sounds and voiceless sounds. If your students say these sounds incorrectly it will create many pronunciation problems.