How do I add files to a BSA archive? The CK didn't add my sounds files automatically so I want to put them in manually. I tried right-clicking the Create Archive window but it does not seem to give me any context menus.

When creating an archive, you can drag your sounds in, and it will add them.

Also, the "Create Archive" option is currently a bit bugged.
Just use "Upload Archive to Steam, then "cancel" when you get past the archiving bit.