Complex sentence: Once Christine sings his music for an audition, she is hit with a sandbag, thus follows a presumed flashback into the past roughly around 1881, where she was the star in the London Opera House.
Split into simpler sentences: Once Christine sings his music for an audition, she is hit with a sandbag. Thus follows a presumed flashback into the past roughly around 1881, where she was the star in the London Opera House.