Zencastr will automatically use the microphone that you have selected as the default microphone in your Operating System settings. You also have the option to choose to use a different microphone.
In Chrome you simply click on the camera icon in the right of the url bar and you will be able to select between continuing to use the OS default microphone or select one specifically. Click 'Done' after selecting an option and then refresh your page. You will now be using the selected microphone for recording.
Firefox will ask in the initial microphone access prompt which microphone you would like to use. If you change your mind or made the wrong selection, you can refresh the page to make a different selection.