As a first step it is advised to ensure that your Chromebook microphone volume is set properly.
Open the system tray and click the arrow > next to the volume slider.
Ensure your microphone volume is set somewhere in the middle of the slider. If your volume is set too high, your audio may clip or break up.
Adjusting your microphone volume should resolve this issue, if not, please also follow the steps in the video below to upgrade your Chromebook to the latest version of Chrome OS to assist in troubleshooting.