How can I turn off the microphone on my iPhone 6S?

The microphone cannot be turned off entirely, however no app has access to it unless you allow it. If you want to revoke an app‘s access to the microphone you can go to Settings > Privacy > Microphone and choose which apps you want to allow access.

If there are no apps listed on that page, no apps have previously requested access.

Hope this helps.

Settings>Privacy>Microphone, and turn off access to whatever app you are referring to.

You're welcome.