I'm pleased you liked my take on the theme! I indeed wanted to add an option to hear the entire scale you built at any moment, but at day four my brain had practically turned to mush so I couldn't figure it out anymore >< And yeah, the visual part is more as a useful extra than the main thing to rely on.
There is a C in level 3, relatively close to the starting point of the level! But in checking, I did find another bug... Still, happy you enjoyed it!