@aimeejensen,
Jeremy is correct; the ability to share audio only is not supported on Chrome.
This is a different support article than Jeremy shared, but it compares all of the features and capabilities of our clients by OS as well as web.
https://support.zoom.us/hc/en-us/articles/360027397692-Desktop-client-mobile-app-web-client-and-PWA-...
If you skip to the “Share Screen Controls” section you will see sharing audio only is only available on Mac, Windows, Linux, iOS, and Android.
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Thanks!