Because AVsync.LIVE is built using web technology, it can only create recordings using Google's .webm video format.
Unfortunately, popular tools and platforms like Instagram, Facebook, and Premiere Pro don't support .webm videos. You'll need to convert them to .mp4.
In the future, this conversion will be built in. Until then, I can recommend these tools to convert your .webm files.
CloudConvert is an online file converter. You can use it free for up to 25 conversions per day.
Any Video Converter is, as you might expect, a free video converter. It's available for both Mac and Windows. It can do conversions very quickly and doesn't require an account to use.