How can I turn on/off audio recording? How do I add sound to my recording?

Mac:

If you are recording on Full-Screen mode you can find the microphone icon to enable or disable audio recording.

On the recording bar Before you start recording, press the microphone icon to enable or disable audio recording.

Windows

You can find the microphone icon on the control bar to enable or disable audio recording.

Mac and Windows

You can also turn on/ off the audio before recording, as shown here. 

You can record sound for HD videos only. If you have recorded a GIF, it will only play without sound. Read more about the differences between a GIF and an HD recording here.