Brightman has undergone vocal training first with Elizabeth Hawes, head of the Trinity Music College in London, and later with Ellen Faul of Juilliard. She currently studies with internationally known voice teacher David Romano. She has a three-octave vocal range that extends to an F above Soprano C.

David Caddick, a conductor of Phantom, has stated:

"What is amazing about Sarah is that she has two voices, really. She can produce a pop, contemporary sound, but she can also blossom out into a light soprano. The soprano part of her voice can go up to an E natural above high C. She doesn’t sing it full out, but it is there. Of course, she has to dance while she is singing some of the time, so it’s all the more extraordinary."

She sometimes deploys both her pop and classical voices in the same song. One example is "Anytime, Anywhere" from Eden, a song based on Tomaso Albinoni's Adagio in G minor. In the song, she starts out in classical voice, switches to pop voice temporarily, and finishes with her classical voice.

Brightman's music is generally classified as classical crossover. According to Manhattan Records GM Ian Ralfini, she is largely responsible for the popularity of the genre. In a 2000 interview with People, Brightman dismissed the classical crossover label as "horrible" but stated she understood people's need to categorize music. Her personal influences include 60s and 70s musicians and artists such as David Bowie and Pink Floyd, and she incorporates aspects of genres from pop/rock to classical and contemporary. Her work has also been compared to that of Madonna, Cher, and Celine Dion. The material on her albums ranges from versions of opera arias from composers such as Puccini (on Harem, Eden, and Timeless), to pop songs by artists such as Kansas ("Dust in the Wind" on Eden), Dido ("Here with me" on La Luna), and Procol Harum ("A Whiter Shade of Pale" on La Luna).