I'll assume we're discussing the Samsung keyboard. Long-press on the button where the microphone button used to be - typically second from the left on the bottom row, next to the "Sym" key. It may currently be a settings-cog icon, or an emoji symbol. You'll see a pop-up offering alternatives for that button. Choose the microphone and it will become the default for that button.
Note that not every application will work in this manner. Many simply do not enable the microphone button on keyboard. For example, the Google app places its own microphone-button on the search-bar, and will not enable it on the keyboard. Most, however, do.
04-09-2016 02:19 PM