Apologies for my delayed response.
So first off I would suggest reinstalling the beta, as it sounds like that helped in the past. Use the following process to complete a basic reinstall: uninstall (delete from Applications), reboot, reinstall.
You can download the installer here: https://download.shedworx.com/jaksta/beta/2.2.6_293/JakstaMediaRecorder.pkg
When you first attempt to run the installer you may receive a Mac OS notification saying that it can't be opened because it cannot be checked for malicious software. Click 'OK' and then go to System Preferences > Security & Privacy > General and click on 'Open Anyway' down the bottom. It will either run the installer or another similar notification as the first will appear and you just need to click 'Open' and then the installer will run.
If that helps with the audio issue but you end up with the zero sized downloads again then do some basic capture tests of YouTube videos. If they work then it suggests you have a capture/download failure issue for the particular site/stream. If that's the case then you can try performing the capture in other browsers as that can sometimes make a difference. Also try out the URL downloader bar in Jaksta, as that is specifically meant as a backup for when the auto-captures fail.
If you can't download any videos without the result being empty unplayable files, then you can try doing a full reinstall. If there is a break in the programs processes then this should reset them.
You will need to carry out the following:
Let me know how you go.
A while ago I had an issue with downloads playing but having no sound.
I can't remember what fixed it but it was either following the support recommended full reinstall, OR installing the latest beta version of the application.
All was fine until recently my downloads refused to open and play and I noticed the OS saw them as having a file size of zero in the finder.
During download the size column in the application window shows a growing file size as usual but when complete that column is blank and I go to view the file on the hard drive and it has no size and cannot be opened.
So yesterday did a reinstall of the current version of the application.
Now I'm back to the original situation. My files download and open and play but with no sound.
I cannot find an e-mail from support mentioning the beta version of the application but I'm sure I remember a conversation with support about it.
I'm hoping someone can help.