First try switching browsers, wi-fi networks or plugging in directly to your internet source via ethernet. If that doesn't help, go to your computer's microphone settings and make sure you have the correct microphone selected and enabled and the input level is working.
Help For Macs:
Go to your system preferences
Then click on the sound icon represented by a speaker:
Click Input. Make sure the correct microphone is chosen and the input volume is set at an audible level (in the middle or above) and your microphone is registering in the input level section.
For more help read this article: http://www.wikihow.com/Activate-a-Microphone-on-Mac
Help For PCs on Windows:
Right click on your speaker icon located at the bottom of your screen:
and select Recording devices
Then click on the recording tab:
Make sure the correct microphone is selected and has a green check mark and the green level is registering your voice.
Under properties you can listen to see if the mic is working, adjust the levels and change the device if necessary.
For more help read this article http://www.wikihow.com/Change-Windows-8-Microphone