When you're trying to use a website like Flipgrid, Vocaroo, GoSynth, or any other site that you want to use to record audio or video you need to grant access to your computer's mic and camera. In Chrome you can do that in the privacy and security settings. You can access those settings by simply typing chrome://settings/privacy into your address bar. Then select "site settings" followed by "view permissions and data stored." Then search for the site that you want to adjust your settings for. Those steps are outlined in this short video.
An alternate way to access these settings is to click on the "lock" icon in your browser's address bar while visiting the website on which you want to adjust camera and microphone settings. That option works the majority of the time, but not always.