How can I make sure the mic input of my laptop/computer/cell is not secretly scanned for keywords and sent to the government?

In order to combat a roving bug you can install a switch to disable your microphone.

You can't. If they want to spy on you they will and it will be difficult for you to find out end of story.

• If you don't want them to hear you through the PC mic drill a hole through it.
• If you don't want them seeing you on the built in web cam cover it up or drill a hole through it.
• Don't use windows rather build your own flavor of linux from scratch.
• They will still find a way to spy on you if they really want to these are very smart people and their job is to spy.

It depends on what level you trust your device. If you trust the drivers, you could simply mute it in the driver and as long as the kernel and driver are secure, it shouldn't be possible for an application to read the input from the mic unless it has kernel level access. If you don't trust the security of the kernel or driver, then all bets are off unless you physically disable the mic.