In this text I will describe a possible fix for on their own working volume buttons.
The problem occured to my iPhones 4S last winter, the volume kept changing on their own up/down, only pressing one button repeatedly fixed it for a short time. So this can either be a software problem (unlikely) or a hardware fault.
I dismantled the iPhone to take a look at the physical buttons. This iFixit tutorial will help you.
The problem was puttygen , that the springs pressing against the flex cable damaged it and shorted it to the frame (GND). You can see the damaged cable in picture 2. This created the unintentional key presses.