Here is the context from a novel 'Angels and Demons' by Dan Brown. I know 'sold' is the past tense of sell, but here, I think the word has another meaning.
Vittoria sounded only slightly more hopeful. "I suppose Galileo could have created some sort of mathematical code that went unnoticed by the clergy."
"You don't sound sold," Langdon said, moving down the row.
Best Answer
"You don't sound convinced" is a clearer expression of the ideas, but sold is a colloquialism.
One trying to persuade another of an idea can be said to be selling the idea. If a person doesn't sound sold [on it] then they haven't bought it, and don't accept it enough to make a commitment.