to me i always thought they didnt say anything because sometimes they have heavy accents and stuff. they would rather make it easier to put the words on the screen so everyone wouldnt be leaving them comments like what did you mean when you said this...or this. thats the way i understood it. sometimes i listen to kids were there accent is so bad you cant understand them and you wish they would put words. thats my two cents and im sticking to it.
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Some people are self conscious about talking. You will notice most of the people who don't talk don't show up on camera except maybe there hands.
The person might be mute.
There camera might not have a microphone or there microphone might be broken.
The whole accent idea.
There are quite a few reasons why someone might not talk during an on-line video.