The external sound card installed is Xonar DG and it comes with SPDIF output header to be interfaced with video card
That was only needed with Nvidia cards from the 8xxx series up to the GT/GTX 2xx series ( I believe starting with the GT/GTX 4xx series it was integrated on the card), as they didn't have native audio on the for HDMI sound. This is what you are referring too, Guide: 8800GT/9800GT/9800GTX Sound over HDMI (Now including Creative cards)
The card you have has its own audio chip on it (Realtek) with no need for the SPDIF connection, and all ATI/AMD cards from the HD 2xxx series and up have been the same. The driver for it is included with the video driver in the Catalyst suite. Once installed it is just a matter of setting the AMD HDMI audio as the default playback device in the Windows audio settings.