It's technically a compressed archive file already!
Viewing post in Black Garden (Alpha 0.1.9b) comments
Well, the problem is, I'm pretty sure I have to compress it using the method available inside the RPG Maker program itself (due to terms of service and the license on some of the music) and that spits out the file that I have attached here.
Basically, the only thing I could do is stick that file in a zip OR extract the game as usual and then stick that in a zip... but you already did that and it didn't work so I don't really know of any other ways to do it.
I also don't really know how joiplay works so I wouldn't really have a clue as to what could be causing the issue with it. :(