Actually, I had the same experience when I picked up Azure Reflections, the side-scrolling STG fangame.
Cirno was the third playable character in it, and throughout my playthrough, I couldn't get my mind off of her very distinct voice. I knew I'd heard it from somewhere, but I put my finger on it. Then I made it to the credits and found out that she was voiced by Yuka Iguchi.
For the record, if you haven't played Azure Reflections to hear it for yourself, I strongly encourage looking up a playthrough to hear her voice. It's fantastic casting, and I consider her voice to be the canon Cirno voice. Yuka's got a lot of VA roles under her belt, but I knew her specifically because she played my favorite character, Hinata Miyake, from my favorite anime, A Place Further than the Universe.
After finding that, I also found out that she voiced Tewi in A Summer Day's Dream. Much less interesting, but it also showed me that Reimu there was played by Mai Nakahara, who plays Nagisa Furukawa in my second favorite anime, Clannad.
So yeah. It was cool to see that everything related to Japan does eventually come around to Touhou if you look hard enough.
Oh, and @CountVonNumenor, you probably noticed, but in Azure Reflections, Rina Satou does reprise her role as Reimu there too. It wasn't shown in the list you provided, but I figured you'd appreciate it.