Have you checked that the volume level is OK when you are playing back your video on the tablet?
Start the video and then press the Volume key to bring up the onscreen volume slider and then adjust the volume to suit?
Oh you meant that you were on a video call, not that you recorded a video using the camera app -camcorder function and there was no sound on playback.
Have you tried using the earphones with the microphone attachment to see if you can be heard better?
If you can be heard properly then there is a problem with the internal microphone. Use a strong light and a magnifying glass to check that the voice 'inlet hole' is not blocked with dirt or lint. If it appears to be do not use a pin or probe to try and clear the blockage as you will damage the microphone which is directly behind the hole. Use a vacuum cleaner, using short sharp bursts to try and clear the blockage.
If it doesn't appear blocked then there is a problem with the microphone.
If you cannot be heard properly then there is a problem with the voice transmission circuitry in the systemboard