I googled for this sentence, but I surprisingly found very few results. Is this sentence idiomatically wrong?
Also, is there an adjective that is related to both "thickness, oiliness" and vulgarity or sexuality, something similar to "Her oily smile" but only related to lewdness?
The context where I want to use this is the following:
The interior was all red, and the women were all heavily made-up. Their voices were lewd.
I would say that idiomatically speaking, no, this is not a viable phrase. I've never come across this. On the same token it is mechanically correct because it's an adjective. You should therefore feel free to go ahead and use it and join the small ranks of other people that you found using it, too.
With regard to your second query, I think the word you're looking for is "husky":
The interior was all red, and the women were all heavily made-up. Their voices were husky.
Google Ngram Viewer shows hits for "lewd voice" and "lewd voices" from 1800 until today, so it certainly isn't unusual.
If you've read enough English literature, you'll find myriad phrases that make sense in context, are grammatical, are semantically acceptable (even if the metaphor is a stretch), and aren't necessarily idiomatic in the sense that they're commonplace expressions. There's nothing wrong with the English, but that doesn't mean that everyone reading it will like it, in which case they will say "It's not good" or "It's not proper" or "It's weird". I don't think it's weird at all. It conjures a clear image for me.
You can probably use smarmy or unctuous or lubricious for "oily" related to lewdness.
Lewd can be used as adjective to describe (for example) photos, remarks, song parodies, laughters, or jokes. If I would read "Their voices were lewd." I would understand that as "Their voices were obscene." probably said in a figurative way.
It's not an idiom, but it is certainly a valid usage.
The phrase the "interior was all red, and the women were all heavily made-up" focuses the reader on the women's faces. "Their voices were lewd" further focuses on the source of their voices -- their mouths. "Lewd" has several connotations. The author is leading the reader but still letting them form their own conception of what a lewd voice might be, and what it implies of the women. I think it's a fantastic usage.
Gramatically speaking, it sounds correct, we are not just assuming that is correct, but it can also be understood just by pronouncing it either from a sentence or a statement.
