AMD High definition audio device is not a soundcard used for speakers, it's actually a part of your graphics card which enables you to get audio when you connect your computer to for example a TV via a port on your graphics card named the HDMI port, you will need a HDMI cable and your TV will need a HDMI port to do this. The soundcard or integrated sound you plug your speakers into I think should be named Realtek High Definition Audio in the device manager. That is how it looks on my computer. I'm not sure why your device manager doesn't show you this. I think Windows should auto detect this sound device. Another idea is to go to http://www.realtek.com.tw/downloads/
and download and Install the High Defnition Audio Codec.
Here is a page on this forum that explains what AMD High Definition Audio Device is better that I can: http://www.tomshardware.co.uk/forum/333084-10-what-high-definition-audio-device