On a lot of sites the video and audio are played separately and likely you only downloaded the video.
In the VDH menu you'll see entries like 1920x1080, 1280x720 etc. That your video downloads. On the audio you should find something
which doesn't have a video resolution but it will have a time matching the duration of the video. That's your missing audio.
The audio is mostly found below the video entries. After that VDH can merge the two files together.
A vdh user