Maniac notes are notes that enhance people's feelings. The latter is because in the general phenomena of poltergeists, they can be heard from anywhere, like a rapping noise for example. The former is because poltergeists in general can move things and made them float.
Since the three sisters' musical performance is something that they have learned themselves after Layla's death, their previous ability was probably along the lines of "moving things without hands or feet" or "performing notes without hands or feet". Since they need not have direct contact with the instruments, they are able to play music impossible to play by hand. By manipulating the spirits of instruments, they're able to manipulate the spirits of notes to create sounds. This probably derives from the ability poltergeists have, put to specific use in performing with musical instruments. In Perfect Cherry Blossom, this is the ability that all three sisters have been specified to have. When she gets interested in something, she gets obsessed with it until becoming almost a maniac Abilities Performing on musical instruments without hands or feet Merlin overflows with happiness and has a very good rhythm, she was never seen depressed.
She also made a cameo in Hopeless Masquerade. Merlin first made her debut on as the stage 4 boss of Perfect Cherry Blossom and appeared as a playable character in Phantasmagoria of Flower View, although she didn't have her own scenario.