The Muses as Underachievers

According to Hesiod's Theogeny, the nine muses were the daughters of Zeus and Mnemosyne, the goddess of memory. The number of muses were classically set at nine; with each having her own area of divine specialization:

Calliope (the 'beautiful of speech'): chief of the muses and muse of epic or heroic poetry
Clio (the 'glorious one'): muse of history
Erato (the 'amorous one'): muse of love or erotic poetry, lyrics, and marriage songs
Euterpe (the 'well-pleasing'): muse of music and lyric poetry
Melpomene (the 'chanting one'): muse of tragedy
Polyhymnia or Polymnia (the '[singer] of many hymns'): muse of sacred song, oratory, lyric, singing and rhetoric
Terpiscore (the 'one who delights in dance'): muse of choral song and dance
Thalia (the 'blossoming one'): muse of comedy and bucolic poetry
Urania (the 'celestial one'): muse of astronomy.

This was the original intention. However, the nine muses, once being quite young, did not consistently apply themselves to their studies as they should have. They were . . . . Olympian slackers. They occasionally spent time in the Olympian Detention, slept late, made poor grades, and did not comport themselves like goddesses should. Mnemosyne worried about her beautiful but unenergetic daughters, but received no help at all from that bounder of their father: Zeus!
In fact, they all turned out okay; but were just not special in the realm of the offspring of the gods and goddesses. Here is what they became:

Calliope, the eldest, was the ringleader of her errant sisters. She became the chief of the muses and was also muse of advertising jingles. She was best known for this one:
Oh, I wish I were an Oscar Mayer wiener
That is what I truly want to be
'Cause if I were an Oscar Mayer wiener
Everyone would be in love with me.
Clio failed Western Civ 101; but recovered and became the muse of the 10 o'clock news.
Erato read excessively of Ovid, Swinburne, and Nin; and became the muse of dirty poetry and song lyrics. However, she is positively regarded for having influenced Barry White.
Euterpe reached her full flowering during the 1970's; and became the muse of disco music. She is stll Stayin' alive; and shaking her now enlarged booty.
Melpomene adopted black garb, fishnet stockings, and a leather choker around her neck, and became the muse of Goths.
Polyhymnia or Polymnia listened to too many Christian redio stations, and became the muse of contemporary Christian music, and is wildly held to blame for Pat Boone's heavy metal album.
Terpiscore went to Vegas and ultimately became the muse of the pole dance
Thalia developed an interest in obvious formulaic jokes, becoming the muse of redneck comedy and inspired Larry the Cable Guy.
Urania moved to Beverly Hills, rose daily at dusk, and became caught in the throes of celebrity worship of such recently-emergent goddesses as Britney, Paris, and Lindsay. She became the muse of Hollywood stars and worked as one of the paparazzi.

Mnemosyne eventually came to accept her daughters, but did on occasion wish that they had achieved more. Moreover, she was disinclined to blame their father Zeus; after all, she too was one of his many 'other women.'