Hey there, thank for bringing this to our attention. This is absolutely something we want to address and thank you for bringing it to our attention.
The music notes give you different sounds when beats of that colour are played. They are purely aesthetic and have no gameplay effect.
Domino is probably our most unusual power note. We've received a lot of questions about it, so we will probably need to make it more clear. If you defeat an opponent with a Domino shot, it will explode and deal damage to nearby enemies, which will also explode if it kills them.