I noticed that when we launch the Music app and none of the tabs is tapped yet, the mix tab is not highlighted. It should be because the first page of Music app is mix page. The trivial way to fix this should be changing the "launch_path" in manifest.webapp, just modify from "/index.html" to "/index.html#mix" then it will work. (Make sure to "make reset-gaia" if you change anything in manifest.webapp)