The internal speaker (top speaker) allows you to hear the person you are talking to during a call. It needs to be replaced on the iPhone 6S / 6S PLUS if it crackles or no longer emits any sound.
This internal speaker at the top generates sound so you can hear the person you are talking to during a call. Here are the symptoms that will let you know if it needs to be replaced:
Replacement is very simple and takes about 10 minutes. No repair knowledge is necessary.