Hey, this is great. I finally got home to try this again and see this is now working. I just tried it playing hulu (not plus) in my Firefox browser, casting the entire desktop and the audio came through. It must be in the latest update because it never worked before, but I'm not sure which update - build, extension, or chrome.
To the OP, what OS are you running? What chromecast build? What Chrome browser version? And what cast extension version?
Here are mine: Win 7 Pro; Build 14975: Chrome 32.0.1700.68 m; extension 13.1210.0.6