Sorry to hear this.
I am way late on a reply for this thread but.....
It is always a hard thing to swollow when you hit am animal and you are not sure of the animal's fate because of the unknown shot....but I guess you did get closure. Better then not finding him at all maybe?
The loud crack that you speak of. My bull last year took two arrows. He was quartering away. The first entered his paunch and blew out the opposite shoulder. I got on the bull again and placed another arrow that entered his back lobe of his near lung and busted his opposite shoulder. He was quartering away slightly and downhill. The second arrow made the loudest CRACK I have ever heard. My guess is because the aeeow hit the opposite shoulder.