Unfortunately, in Zoom on a Google Chromebook, there's is not a volume bar like we'd typically see to adjust our speakers. In Zoom, however you can adjust your microphone settings. 

On the bottom left-corner of your screen, directly next to the microphone icon,  click on the Upward-facing-arrow. Here you'll be able to select a microphone. By default, the Chromebook will select it's internal/laptop mic. At the bottom of the menu, click on Audio Settings

Next, in the Audio Settings window, click to select Automatically adjust microphone volume. Then, click the X to close when finished. 

If you are still having troubles with your Zoom microphone/audio, please email support@kwcs.freshdesk.com

NOTE: If you have corded headphones with a mic built in (for example corded headphones that came with your cell phone, if they have a mic built in)  will work in the headphone jack, on the side of your Google Chromebook.