You will see separate audio devices in your computer for the MB on-board and AMD HDMI. Simply ensure that you have the latter selected as the default output device (whether as Windows default or from within your media player).
Remember to set up your speaker configuration (stereo, 5.1, 7.1, etc; large-/small-sized speakers on individual channels) as well. I believe Windows defaults to a Stereo set-up so multi-channel system owners need to do a little setting adjustment.
Cheers!