How to disable sound button?

Settings>General>Use Side Switch to>Lock Rotation. that will turn the side switch into the switch that locks the screen orientation instaed of the system sounds.

I'm not sure that this is exactly what you are looking for but that is how you would disable the switch from controlling system sounds. Now the system sound control will be all the way to the left in the task bar at the bottom of the iPad when you double tap the home button.

If your problem is that the cover is pressing the volume rocker, there is nothing from a pad perspective you can do. But an exacto knife is a wonderful thing. Over the years, I have 'modified' an awful lot of covers, and that may be the solution for you in this case.