@djhalp - There does not appear to be any sound notification setting in Miro itself. I presume you are referring to using Miro through a browser. If through Chrome, you can modify website specific sound settings as following:

Chrome → Settings → Advanced → Content settings → Sound → Muted → (input your URL)

And pasting this into the Chrome address bar may get you right to the above:

chrome://settings/content/siteDetails?site=https%3A%2F%2Fmiro.com%3A443&search=sound

This is the only article I could find in the Miro Help Center re notifications in Miro → Miro Notifications