itch.io

Just launch the demo. What is keeping you? Do you even intend to release the game on Itch? You can upload an in-development version on Itch and even charge for it. Plus upload a demo version on top. That's early access at other places.

You think more people will visit your demo page, if you do not release it? Or even remember your page later, if they find it now and there is nothing to play? You will see interest of that kind by the number of collections your page is in, and by how many followers you have.

That's just my opinion. You do whatever you think best for your marketing. I am not a marketing expert, just a user. A community moderator already told you, that uploading a file that is not a game and calling it a game can lead to bad user experience.

From my viewpoint you are unknown. You have zero released games. Drumming up a release of ... a demo, not even of a game, is not a thing anyone is waiting for. And if I were to see a demo and could not get access to the full version, I would be disappointed. In your case, there is even one further step of abstraction, as I could not even play the demo, because you only published bonus material. As I said: no one is waiting for a release of your games, because you are practically unknown and the people that do find you, cannot play any samples of your work.

Also, you might want to tell potential customers your plans for releasing the actual game. Will it come to Itch, will it be on Steam, will you give a Steam key with the Itch purchase and so on. https://itch.io/games/steam-key

So then how would someone upload a downloadable soundtrack or artwork before launching a demo?

Launching a demo. Just "launch" the thing already. Better yet, launch the early access version of your game, complete with option to buy it. You did read the docs, did you not? https://itch.io/docs/creators/quality-guidelines#avoid-only-uploading-keys-or-li...

And if you sell a game here, you can easily set a release date and have your project to attach files to and demo files, an early access price, devlogs and whatnot. I initially assumed you are under obligation from Steam to have global release date and not release early on other platforms. But this game is not even on Steam wishlist. So I have no clue why you wait to release your unfinished project. Unfinished projects is common on Itch. There is even protoypes that collect money. https://itch.io/games/top-sellers/prototypes and of course  https://itch.io/games/top-sellers/in-development  

In metadata of yor project:

Release date — In your local time zone

Leave this blank unless you have a separate release date from when your published your page. Changing this date will not affect how your game is displayed on our popularity and new-releases pages.

If you set this date in the future then we'll restrict purchases until the release date has passed.

There are many options to release files on Itch. But what will users expect. You can release the soundtrack of your game as an extra project. https://itch.io/soundtracks Will that garner interest in your game? I have serious doubts. You have nothing that can be played. Any interest you garner will lead to a wall and with the short attention span of people you will waste that attention if you cannot give them anything to try. 

Itch is welcoming to projects that are still in development, you might want to embrace that.