Thank you @Baillie.
I expected it to be in "Updates," but that was empty. Per your note, I started to re-download the whole of Duality Strings from "Not installed." (Needless to say, it was already installed. In fact I've got 80 "Not Installed" entries and they're all installed.)
So once the "Not Installed" Duality Strings started to install, only then does "Updates" show up with its own "Synchron Duality Strings," which is only 8.4 MB -- hence the likely free expansion sounds.
Unfortunately, I'm stuck in the download of the whole library, and I'm not going to interrupt it.
Thanks again for your post.