ATI is the graphics card developer. Your motherboard has onboard audio that appears to be Realtek. You should not have an ATI sound driver... as they don't exist.
I'm assuming you're using WinXP to make thing simple. If you're using Vista, try to do the equivalent.
Since your graphics card is capable of HDMI, the problem could be that the card expects to output audio through your graphics card rather than the sound card. Check start -> Control Panel -> Sounds and audio. -> Sounds and Audio Devices /or/ Adjust Speaker Settings -- go to the Audio tab and make sure it says Realtek for output. If it shows more than one output device, set it to realtek. If it doesn't show realtek, follow the below instructions, and then double check at the end that the Output device is set to Realtek.
1>Please go to HP's site
and download their Audio drivers.
2>Go to Device manager -> Sound, Video, and game controllers.
2.1> Does it have an entry saying "Realtek blah blah Audio" there? If not, what entries does it have for audio? Ignore Audio Codecs and Legacy Audio Drivers.
2.2>If there's nothing there, you need to install the drivers you downloaded. If there is something there, please continue to #3.
2.3>After installing the driver, restart computer.
2.4>Once computer is restarted, go start -> Control Panel -> Sounds and audio. -> Sounds and Audio Devices /or/ Adjust Speaker Settings. Hit the button labeled Advanced and adjust the speaker setup to whatever you have - surround sound, 5.1, quad, stereo, whatever.
2.5> Go to #5
3> If there is anything that says Realtek/Azalia Audi, right click it and Uninstall.
3.1> Restart computer.
4>Install audio driver you downloaded, and perform #2.4.
5>Find the driver icon (there's probably on in the task tray) or find it under start -> Control Panel -> Sounds and audio. Set your speaker settings there as well. Audio I/O should specify what ports go to what speakers What you set in windows should match the Realtek audio manager's speaker setup.
If problem persists, please report this.