Unfortunately the way Hangouts is coded and the way the Mac machines operate, they sometimes don't get along very well. If you've recently purchased a Mac, or updated and are seeing microphone problems, this could be the solution to your issue.
Step 1- Open your system preferences from the Apple logo in the top left of the screen.
Step 2- Click on the sound icon inside the new window.
Step 3- Select the input tab on the top of the new window that opened.
Step 4- Make sure the correct microphone is selected, in most cases this will be the built in one, unless you are using an external headset.
Step 5- Un-check the tick box next to "Use ambient noise reduction", directly under "Input level".
Step 6- Do a full reboot on the machine.
This will allow the computer to detect the settings that were changed. Then test out the microphone in a hangout with a friend, or give us a call and we'll be more than happy to test with you as well. The majority of first-time Hangouts on Mac users will experience this, but it's a very simple fix.