I tested the Dolby Experience demo page in Chrome and got the same message directing me to use Edge for the Dolby-encoded audio. The volume control is grayed out in the video player unless you use the normal Edge browser.
From what I could find, about six weeks before the official release of Windows 10, Microsoft announced that "Microsoft Edge is the first browser to include support for Dolby Digital Plus." That support now extends to Dolby TrueHD and Atmos, but as far as I have seen, Edge is still the only browser to include this support.
Sorry, just to be clear, you've tried both (old)Edge and (new)Edge Chromium?