Hi there,
I had this same issue; I accidentally dropped my headphones, and the volume button stopped working, what I did to solve it is opening them and as you can see in the picture I have attached, the two little buttons were broken, so. I manually used a screwdriver to provide support in the back and with the headphones turned on, raise the volume to the max limit. Now I have to control the volume from the source, phone, computer, etc.
[image|1806927]