I'ver recently updraded to F15 with gnome 3 and am also trying to use gmail to make phone calls using a usb handset. I can go to system settings/sounds and see the handset and do a test sound and it works, however, when I go to use gmail to make a call all sounds are from the internal speakers of my laptop. It doesn't see the handset). This is a real problem for me since I don't have a built in mike.
In earlier versions there was another setting where you could select the default sound input/output devices.
Where might I find these settings and/or does anyone have any other suggestions?