@@user-ss1tc1pj7v Nana recently posted a video of her singing in a karaoke and you can tell there that she really has a good voice. she's sitting but was able to reach the high notes of the song. you can check it out too
if you own a set of headphones put it on your head and while listening to music take it off, you'll notice the pitch has changed. so they wear headphones to know exactly in which specific pitch to sing. pro's do not need it cause they have a better musical hearing raina is a true singer unlike nana and lizzy. so she doesn't need one
diana nati Raina does wear the headphones in some parts, and honestly I think I know why from experience if I sing I can't do it with headphones or at least one ear on and off since I find that I need to hear my voice's pitch.
As a musician i can tell that those headphones work as monitors, they hear both, music and their own voice (also other girls voices if they want to), they tell the guy in charge of the controls how much music or voice they need in their monitor until they are comfortable with the mix.