In order to select your USB headset as the default device on a Chromebook; click on the avatar icon in the lower right hand corner next to the time.  This should bring up a menu that has an icon that looks like a USB plug.  Click on this icon (marked in red in the screenshot below):
 
This will generate another menu that lists your input (microphone) and output (speaker) devices.  Make sure that there is a check mark next to your USB device for both input and output.  If it is not checked, click on it to select it as the default device.