Here's a quick speech lesson of the day. Don't you always want a speech lesson on Mother's Day? (Happy Mother's Day everyone!)
Say, "Ssssssss" out loud like you're making a snake sound. Draw it out as long as you can and while you're doing it place your hand on the front of your throat near your adam's apple. Now say, "Zzzzzzzzz" out loud like you're making a bee sound. Draw that one out as long as you can too while keeping your hand on your throat.
The first thing you should notice is that your throat vibrates while you make the /z/ sound, but it does not while you make the /s/ sound. That is because /s/ is a voiceless sound. You can make the sound without vibrating your vocal chords. The reason you feel your throat vibrating when you make the /z/ sound is because it is a voiced sound. You have to vibrate your vocal chords to make the /z/ sound. Other than that one difference, voicing, the /s/ and /z/ sounds are made in exactly the same way. You raise your tongue tip near the roof of your mouth behind your teeth and blow. So /s/ and /z/ are a voicing pair. They are two sounds made in exactly the same way except that one is voiced and one is not.
There are lots of voicing pairs. /t/ and /d/ are voicing pairs. /t/ is voiceless while /d/ requires vibrating your vocal chords. /p/ and /b/ are voicing pairs. /p/ is voiceless while /b/ requires vibrating your vocal chords. /k/ and /g/ are another example. /k/ is voiceless while /g/ requires vibrating your vocal folds.
What does any of this have to do with apraxia? Well, making a voiced sound is a more complicated motor task. To make a /b/ you have to do everything you have to do to make a /p/ and then coordinate vibrating your vocal chords at the right time for the right duration. So often, children with apraxia will find voiceless sounds easier. /t/ and /p/ are often easier than /d/ and /b/. Just another example of how complicated the motor planning of speech is and why our children sometimes seem to have trouble with a sound or word for no reason when there really is a reason after all.