It is possible that in some cases a video may not have audio but it is also quite possible that it has audio but your video player doesn't have the codecs it needs to open and play the audio. This is especially true if you are using the free Version of Quick Time.
If you are using the free version of QT as your video player, please do the following to download a video file in a format that QT can play.
Open the Sniffer or YT module and go to the video of interest.
Click the green DS4 button and allow the list box showing the videos to appear.
Next, click on a ROW in this list box (it will become highlighted) and then click the BLUE download video button.
This
will open a dialog window. At the bottom of this dialog window you will
see the format as mp4. Change this from mp4 to .mov (QT format). The
proceed with downloading.
Once this file is downloaded and saved as a .mov you should hear audio when you play it in QT