If memory serves, the error you are having should be bypassed when you right-click on the executable and select "open archive" through the 7-zip context sub menu, that should work. Let me know if it still doesn't though. Let me know if it works as well.
As I mentioned, the only thing that is inside of the archive when I open it via 7-zip by following the steps you mentioned is the audio/bgm file. I have tried this several times, redownloaded the .zip, and had others attempt it as well with the exact same result. I have attached a screenshot with what I see when I open the archive via 7-zip as well as a screenshot of the error message I receive when trying to extract it via 7zip. As you can see, there is nothing in this file.
