FYI: The ticket is under review here: https://fvdmedia.userecho.com/topics/6818-no-sound-recorded-with-mp4-video/
If you can please provide the developer further data like URL-s of the videos you have downloaded, what OS you are using etc. The more input there is the sooner it will be fixed.
P.S. I'm not the developer but just a regular FVD extension user. I noticed your message and decided to help you with directing you to the main ticket.
Customer support service by UserEcho