By default, the mic is ON, on these smart speakers. But you might accidently turn off the mic. Then, with the microphone OFF, Alexa won’t hear you call her. So to get her to listen again, you must turn on the microphone on the Echo Dot 3rd Generation. Thus in this post, we explain how to do that.
How to Turn On Microphone on Echo Dot 3rd Generation
So if the mic is OFF, the light ring on the Echo Dot 3rd Generation speaker glows red.

But if there is no red light, then the microphone is already on. If it isn’t though, you can turn it on by pressing and releasing the Microphone On / Off button. E.g. Find this button in the last picture in the nine o’clock position on the top of the speaker.
When you press this button, the red light ring shuts off. Then the speaker makes a low plunk sound. Now with the mic not muted.

Finally, all buttons and the light ring are dark as Alexa is standing by. So it is now ready to respond to your voice requests. This means that you have now turned on the microphone. Thus you can now test it by asking it something. E.g. “Alexa, what time is it?”