How to Disable the Camera Shutter Sound on an HTC
On some HTC models disabling the camera shutter sound can be a convoluted process, but it is still effective. From your camera app, hit the three dots (hamburger) icon, then the cog wheel icon and go to General settings > Camera shutter sound. Untick the box.
On the newer HTC 10, it is the same as for stock Android devices. Just change your master volume settings, and the shutter sound will be silenced.