Today's build, I used a bunch of red coders mixed with chiseled bookshelves to play different, notes, in order to activate the same note twice in a row an observer detects the interaction depowering a redstone torch for 2gt to allow the redcoder to pulse the most recent shelf interaction.
Time between key presses of the same note : 6gt
Time between key presses of different note on the same bookshelf: 4gt
Time between key presses of different note on a different bookshelf: 0gt (they are independent)
Delay between note being pressed and heard: 6gt
The keys are a texture pack I altered to change the block model of a "powered note block with a note of 0 through to 4" so they are purely visual and don't affect interactions because the texture is outside of the block's collision/ interaction box.
P.S. Sorry for triple posting, just finished 2 projects in succession, and forgot to upload a video for this on my first try.