My native speakers say the construction "không nghe thấy" is perfectly valid, but it adds an extra connotation to the verb. One would use this phrase over simply "không nghe" when the act of not hearing is because the person "bạn" or what they are saying is disagreeable. so the English translation for this sentence is rather simplified, but still technically correct.
English translations often seem too approximate. Here the word noi is eliminated. The literal translation should be : Sorry, I didn't hear you say! and not Sorry, I do not hear you! . For this last one, my Vietnamese colleague tells me that the translation is : Xin lỗi, tôi không nghe thấy bạn! Who is right?