That may be your problem.
Normally Windows 7 DVD will provide most drivers, but if some device doesn't work, the you go to your computer manufacturer site to download missing drivers.
Windows 7 installs its own generic drivers, which may, or may not work.
In your case, HP site is of no help, since there are no drivers for Windows 7.
As Allan mentioned before, the most important is a chipset driver, on which many other drivers (like ethernet, audio, etc.) depend.
If the audio is the only device, which doesn't work properly, you have couple of options.
You can try to find out model of your motherboard and see, if there are any Win 7 drivers available, or buy a separate sound card.
I can see from here:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00590499&tmp_task=prodinfoCategory&lc=en&dlc=en&cc=us&os=4063&product=1847041&sw_lang=, you have ASUS P5LP-LE motherboard.
I went to Asus site:
http://support.asus.com/download/download.aspx?SLanguage=en-us, but I can't see your specific motherboard model.
We may need more info.
Download
System Information for Windows:
http://www.gtopala.com/siw-download.html to your
Desktop.
Get
SIW Standalone (English-Only).
Double click on
siw.exe to run the program.
Go File>Create Report File>HTML
Save the file to known location.
Zip the file, and
attach it to your next reply.