If the said button switch is okay seems working but still unresponsive you may then consider that the problem may lies on a faulty circuit components on the PCB board.
This solution below shows the line paths of each particular button switches of the camera, slider lock and the volume up and volume down buttons.
Ensure that the button switches is perfectly clean and working okay.
There is an EMI-ESD filter IC is connected to each button switches line paths., Check this filter IC and trace the corresponding button switches line paths first before trying to replace it.
The Volume up control switch is connected directly to the I/O expander chip, you may check and rework this
chip if only the volume up is not working without touching the ESD filter IC.