Don’t bother visiting mikumix.com. It’s long gone and if it connects you, it’s something different entirely. I’ve linked where the wallpapers were archived.
But if you're curious...
There’s a computer program called Vocaloid developed by Yamaha that quite literally sings. You could buy voices for the program made by other companies. One day one such company thought: “How could we increase sales?” And so they put an anime girl on the cover…
Hatsune Miku is recognised as THE most famous Vocaloid voice ever and there have been actual live concerts of her. She’s notoriously not from any anime. So one time there was this Flash website made by some fan with Miku content and mesmerizing background music. And there were some pretty wallpapers there.
It’s been gone long before Flash died, but the images survived, copied from one wallpaper site to another, credits lost to time. Perhaps you’ve seen some of them in the wild (I’ve seen one in a physical shop on a mouse pad). Keep one of them alive on your desktop, if you’d like.