Sentence:  I had squeezed the trigger of my own gun , and to my amazement , it had fired and kicked back into my shoulder with the force of an angry mule ; and then I was <WSD>adding</WSD> my own voice to the crescendo of sound , hurling more vile language than I ever thought I knew , sobbing and shouting , and aware that if I had passed water before , it was not enough , for my pants were soaking wet . 
Meaning of the word marked with <WSD> tags:  make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of