We had the song for a while ever since volume 9 dropped, though you just have to do some digging to find it. I have the physical disc. Perhaps Anon could've added a scan of the cd as well, but you can find it in this video. Much thanks to Anon for such great work.
There is also this person who synced the song with the chapter (some spots aren't timed right, but the majority of it is good):
(if this isn't showing up for you, click "watch on youtube").
Ooooh right, her name is also Ichikawa. I thought for a second our boy got a random gay admirer in the last chapter. This makes more sense.
Good for sis, I guess.