Link1: Downgrading Hewlett Packard dv6000 series Notebook from Vista to XP
Link2: The Official DV6704nr "Vista to XP" Guide
Download and Install the drivers as mentioned in any of the above link.
Even after following step 5 as described in above Link1, if 'Audio Device On HD Bus' doesn't show up in device manager, you need to follow step 3 of Link2.
But neither step 5 of Link1 nor step 3 of Link2, helps in resolving 'Audio Driver' issue in Window XP with Service Pack 3. To resolve it, follow the steps as mentioned below.
1. Open 'Start' -> 'Run'
2. Give 'regedit' command to open registry editor
3. Modify Value of 'CSDVersion' key in 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Windows' from 300 to 200
4. Restart machine
5. Install KB888111xpsp2.exe
6. Modify Value of 'CSDVersion' key in 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Windows' from 200 to 300
7. Restart machine
These steps are also found here
Now, you will have 'Audio Device On HD Bus' in device manager. Now, you can install 'audio driver' on this device.
Steps mentioned above are tested on HP Pavilion dv6707 with WinXP SP3. They are working perfectly fine.