If the contact is prolonged (a carry), I should have a visual AND maybe the sound. I will agree that there may be a high correlation between carries in the chest/arm area and that distinctive sound, but it's not 100%. And what if I don't see the contact but hear it? Do I call what I think happened? Long sleeves are back in fashion for volleyball (but please, please don't go back to the buns), and they can hide any potential sound. And some of those balls that make a sound when hitting the upper chest area are really a clean double rather than one long contact (again, angle is everything and sometimes I have the worst angle), so it's back to what I see.
I teach newer officials that we don't call things we don't see. Don't do it, it will cause more problems than it's worth in the long run. I'll take any grief I get for not seeing a call a hundred times over before I want to take grief for making a call on what I heard but didn't see. Also, there are techniques the R2 can use to make suggestions to the R1 to help with calls the R1 cannot see (this could be a long thread all by itself), but we should only call what we see (or get help with from the R2).
Love the threads out here, hope this helps.