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Her singing voice is highly expressive.
Literal
Her singing-voice [topic-は] very expressive-power rich is.
表現力豊か breaks into 表現力 ('expressive power') and 豊か ('rich, abundant, plentiful'). The construction X豊か(だ) = 'rich in X' is a productive frame — you'll see 緑豊か ('lush with greenery'), 個性豊か ('full of personality'), 経験豊か ('rich in experience'). It's a slightly elevated way to praise quantity-or-quality of an attribute, common in writing and reviews. Combined with とても ('very') for added emphasis, the sentence reads as a polished compliment of the kind you'd find in a music review.