The full sentence should be "空山不见人，但闻人语响。", in which “响” is the simplified version of "響".
Obviously, it should mean "sound or echo" for in the first sentence the author said that he did not see anybody in the mountain and in the second sentences there is a turning point marked by "但", which can be understood as "but". After this turning point he said he could hear the "sound or echo" of words of people.
(You can replace "sound or echo" with "want" and the sentence will not make sense if you do)
I guess you may confuse "响" with “想” as their sound are the same.
By the way, I prefer "echo" than "sound" as he did not see the people but just caught the words in the wind and this poem tends to create an atmosphere of serenity.