By default, these chromium-based browser will not try to output more than stereo channel to your sound system. I've tested them myself with Edge. Turns out, you need to enable them to do it via passing some parameters command line through a shortcut of the app itself.

To do it, you need to do the following things:

Locate your browser folder. For Edge, it is usually on "C:/Program Files (x86)/Microsoft/Edge/Application". Now you copy the path.
Navigate to your desktop, then right click, and select "New > Shortcut". It then will open a "Create Shortcut" wizard.
On the "Type of the location of the item" input field, enter the path we copied before. It should look like "C:/Program Files (x86)/Microsoft/Edge/Application/msedge.exe". now we add this command line parameter "--try-supported-channel-layouts". So the whole thing should look like:

"C:/Program Files (x86)/Microsoft/Edge/Application/msedge.exe" --try-supported-channel-layouts

Click Next, and for the name, it's up to you. I call them "Edge 5.1 Enabled". then click finish. Now it should work.

Enjoy streaming multichannel content on your sound system now!

If you're system does support 5.1 audio but only from Dolby Digital (not from PCM directly) you're in for a treat! Because I happened to just have the solution for you here.

Enabling Multichannel, Surround Sound Audio Support on Chromium-based browsers on Windows (Edge, Chrome, etc.)