What word(s) describe a woman's voice as "sexy"?

maybe "Sultry"? It's not specifically used to describe a voice, but it's a common use.

(of a person, esp. a woman) attractive in a way that suggests a passionate nature.

You could say she has a sultry voice.

See entry 2b:

exciting or capable of exciting strong sexual desire

"A seductive voice" sounds good.

A husky voice?

Husky voice is the key to seduction —headline in Hindustan Times

Husky has the specific connotations of deep and alluring that you’re asking for — though maybe not so much of smooth.

Moving beyond simple adjectives, there’s a long tradition of using similes, especially food and drink, for sexy female voices:

• like honey
• like Bourbon
• like (melted) chocolate
• like melted butter
• like (fine) (red) wine
• like velvet

These are especially popular for describing jazz and soul singers singers — I remember some Nina Simone and Ells Fitzgerald liner notes from the 60’s and 70’s with great examples, which I wish I could find online…