The sound has a very noticeable "t" sound in the middle of "vend-il", making it sound almost like "vend-t'il". I don't know if its supposed to be there, or if it's just bad computer speaking.
Yes, it is indeed supposed to be there. That is a liaison between two vowel sounds: en-il which would make difficult to say and difficult to understand. That is why the final "d" moves to a "t" sound in front of a vowel.
Given the distinctive "t" sound in "vend-il," why isn't "Combien le vendent-ils?" a valid answer?
it needs to be "how much does he sell it for?" You would be understood if you said it your way, but it's foreign -- no native would say it that way.