One of the wonderful things about poetry (and song lyrics, which are another version of poetry) is that the author as well as those who read the poem aloud (or sing the lyrics) have some freedom about matters just like this.
It is the meter of the poem which determines whether a three-syllable word should be pronounced as a two syllable word--or vice versa. If the poet needs three syllables to meet the metrical requirements of his line, then he wants it to be read as See-lee-a. If he only needs two syllables, then the poet prefers the word to be pronounced Seel-ya.
Celia\ce-lia\ as a girl's name is pronounced SEEL-yah. It is of Latin origin, and the meaning of Celia is "heaven". From Caelia, feminine form of the old Roman family name Caelius, probably from "caelum" meaning "heaven". Sometimes also used as a short form of Cecilia.