No, not a second gunshot. I didn’t know if the bang was outside or in the apartment. If there are other 911 calls at the same time that will be clear.
Here is a list of transcripts, not sure it if it includes John’s.
http://news.blogs.cnn.com/2012/03/20/911-calls-paint-picture-of-chaos-after-florida-teen-is-shot/
The best video and best evidence is at the link above or below. Better than any 911 call recording because you can't be sure what you are hearing on those.
Video at upper left corner has the reporter standing on the spot where Trayvon was shot. Shows you from there just how close the windows that eye witness John saw and heard the event. They interview John on air and he tells what he saw and heard.
This is by far, is the best evidence in defense of Zimmerman. He saw and heard it up close. You can see for yourself just how close he was to the event. No other witness has this perspective.