Tom is the mockingbird in Chapter 15. Heck and Atticus feel it would be safer for him to be in the jail, and Atticus even goes down to the jail and sits there to help guard Tom while he's sleeping. The Sarum Bunch shows up that night and wanted to take Tom with them and lynch him. Even though he did nothing wrong and nothing has been proven yet, these men want him dead. A mockingbird does nothing wrong. All it does is sing pretty music for others. Tom did nothing to anyone--least of all these men from Sarum. All he tried to do (as we find out later) is help Mayella from time to time.
The mockingbird in Chapter 15 of TKAM is Scout. Jem and Scout receive air rifles for Christmas in Part I chapter 9. In Chapter 10 Atticus tells them they can shoot anyting they want except for a mockingbird. "Shoot all the bluejays you want, if you can hit'em, but remember it's a sin to kill a mockingbird." When Scout asks Miss Maudie about the only thing Atticus has ever said was a sin she responds: "Mockingbirds don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." Since this question is addressing Chapter 15 only and at this point the reader would not have knowledge of what happens later in the novel, the evidence lies in Scout's speech given to the mob. She is an eight year old little girl, a tomboy, who reminds Mr. Cunningham he has children too. Like a mockingbird she is "singing her heart out" trying to find a subject with which to engage Mr. Cunningham, the only familiar face in the angry mob, in conversation. She does this innocently, like a mockingbird. And like a mockingbird with an air rifle pointed at it, Scout does not realize she is in danger.