There are two ways to begin and end a voice-over recording. 

You can select the microphone icon in your Reflector Teacher menu. The drop-down arrow beside this icon will let you choose which microphone you will record from. This must be a microphone that is attached to your computer.

You can use also use key commands to begin and end a voice-over. 

Mac: Press Command + Shift + V to begin or end recording voice-over audio.

Windows: Press Alt + Shift + V to begin or end recording voice-over audio.