In my mind, I think Haku's life story went like this:
Haku was a bright, cheerful girl with a beautiful voice. She practiced for her coming auditions, succeeded, and was a well known singer until the day her vocal chords got damaged due to the constant stress she put upon them. This sent Haku down the spiral of alcohol and depression.
Even though Haku retains her voice, she does no longer have the melodious singing she once had.
And then came her younger sister, Miku. She is a reflection of what she once used to be.
...And it is slowly killing her.