If you are using a MAC with a Chrome browser and don't get any audio in the Studio audio mixer after enabling the audio return. You may need to use a custom Studio version available here:
Add the following version at the end of your Studio URL.
?version=2bd333b2b7219bc524ca5af471fa086fd3fc697b?version=2bd333b2b7219bc524ca5af471fa086fd3fc697bYou should now have audio in your Studio Chrome tab.