Reading audio buffer data with AudioQueue

Answer 1

char *buffer= NULL;

That's the reason. You're iterating over signed bytes, not 16-bit samples.

Declare the variable as holding a pointer to two-byte values instead:

SInt16 *buffer = NULL;

Then, iterate over half as many samples as bytes:

for(int i=0;i < (BUFFER_SIZE / sizeof(*buffer));i++){
  NSLog(@"%i", buffer[i]);
}

I would rename BUFFER_SIZE to BUFFER_SIZE_BYTES to clarify it.