You need to download the audio driver, and install it.
Just had to do this with my Gateway after a Restoration.
This is HP Support, and the main support page for the Pavilion a730n desktop computer,http://h10025.www1.hp.com/ewfrf/wc/product?product=431078&lc=en&cc=us&dlc=en&submit.y=5&submit.x=1&lang=en&cc=us
On this page left-click on the heading -
Software and Driver Downloads. (Ampersand for and)
On this page choose your Operating System.
I chose Windows XP for an example.
(Left-click on the blue down arrow to the left of the box, that has Select in it. Then left-click on the Operating System your computer is using. Now left-click on Next)
Left-click on the + sign to the left of -
Driver - Audio (1)
Now left-click right on the file name -
Realtek High Definition Audio Driver Update
Left-click on Download.
(In blue rectangular box with two right arrows)
Instructions from HP Support for downloading, and installing:
1. Download the driver update.
2. Double-click the icon for the update file downloaded in Step 1, and then click NEXT
3. Accept the terms in the HP license agreement, and then click NEXT
4. Click YES
and wait for the update to complete.
5. Click YES
to restart your PC.
Once you have restarted your computer, and Windows has full loaded, you should get a pop-up out of your system tray.
Your audio (Sound) should be back now.
If this doesn't work the only alternate suggestion I may offer, is to but an inexpensive sound card, and install it.
(Computer unplugged from power, follow Anti-Static Precautions.
Don't forget to install the CD disk from the sound card FIRST, then install the sound card)
If you have additional questions regarding this, please post in a Comment.
(Would like to hear from you so that I know you read my solution, thanks)