Make sure that the microphone icon is selected at the bottom. It will be highlighted blue when it is active.
The Megaphone being active (blue) will broadcast your voice across the whole frame. Without this selected, you will only be heard by other avatars in your proximity:
Try a hard refresh by pressing Ctrl + R
Check the microphone in Audio/Video Settings with the gear icon on the right of the action bar at the bottom of Frame:
Check that Mic permissions are enabled in Chrome with the menu button on the left end of the address bar:
Try an incognito window:
Incognito Mode / Private Browsing
If you are using Windows, check the audio settings in Windows Sound Settings:
Adjusting Sound/Recording Settings (Windows)
Make sure the browser is updated:
Updating Google Chrome
Updating Safari
Updating Firefox
Updating Microsoft Edge
Try a different web browser.
Make sure there are no pending system updates: Updating Windows Updating Mac