If your recorded video has no audio/sound it could be caused by a number of issues.
Try these tips to get sound back in your videos.
Check the volume - make sure the device's volume isn't set to zero
Check the Ring/Silent switch - make sure it's not set to Silent
Check the case - some phone cases can cover or obscure the microphone
Disable Bluetooth - connected Bluetooth devices may cause silent recordings
Update your OS to the latest version - this can fix odd software glitches
Restart your device - this can also fix glitches
Some ways to check your microphone is recording properly:
Record some audio in the Voice Memo app to make sure the audio works
Record a video in the Camera app to ensure the video has sound
If you still have issues with silent video recordings after trying all these tips please get in touch!