Very nice!

I've worked on a small toy project with a similar purpose in the past [1], though it's not nearly as polished as yours, and I've made some questionable decisions here and there.

I have questions about pitch tracking. It seems you do track the pitch for scoring, and there's a line at the top of the screen that seems related but that I can't figure out. For my use case, an important feature of karaoke apps is displaying how "high" the next note should be sung, or at least some hints. Is it something your app can do and I just haven't figured it out? Or would it be a feature request?

[1] https://github.com/eckter/karaoke_helper