What purpose does 'thì' have in this sentence? Does "Khi tôi hỏi bạn trả lời?" not also mean the same thing?
it's just a sentence structure (khi...thì...) that differs from English
I don't hear fee. But i am german and we have a "ch"-phoneme similar to the "kh"-phoneme in vm. I can imagine that it must be difficult to hear it or even speak it if you don't have such a phoneme in your original language.
This is better translated as "When I ask then you answer." English speakers of simplify the sentence by omitting "then" (and using a comma like a conjunction) but this is a less complete sentence. It actually translates directly.