The audio goes to the selected PLAYBACK HARDWARE on the sound option at the control panel. If you select ATI-HDMI audio, the sound goes to the 4670/HDMI. If you select the on-board audio to sound will come out from teh 4670.
I have CF 4890 and Sound Blaster XFI Fatality Audio card. If i want my audio to go the my TV through the HDMI port i select the ATI-HDMI as the playback hardware. This is done on the sound option of the control panel.
If i want the sound to go to my receiver i select SPEAKERs on the SOUND OPTION at the control panel.
To summarize, both audio hardware (on board and 4650) will work. The audio/sound will go to the one that you select at the SOUND OPTION of the control panel.