Did you try the drivers I linked to? Also, make sure you uninstall any old sound drivers before installing new ones.
Google pointed your model number to ASUS. After navigating around the ASUS web site, I managed to find
this. It lists two motherboards which share nearly identical model names to yours (VM and MX instead of the VX yours has).
They both had a downloadable "overview" PDF, one of which lead me to Sound Max. You didn't mention Sound Max before, but it's still possible. But as I found out years ago with a Gateway, Sound Max simply redirects you to your OEMs site when you go to drivers. It also directs you to motherboard manufacturer's, so back to the ASUS web site I went.
I managed to find a few pages which contain driver updates that may or may not be for the motherboard your Sony uses. You'll have to navigate to them yourself, but you can find the page that lets you selects the model number
here. Select Motherboard, then Socket 478, then each of the following models:
P4S533
P4S533-E
P4S533-X
P4S533-VM
P4S533-MX
P4S533-X
The VM and MX, as I said, use the same chipset as yours so it's quite possible that yours
is one of them or is very similar. Click the Drivers tab after you navigate to each one and find the sound drivers. I saw AC97 and C-Media, but I didn't look at all of them. It's quite possible that your motherboard is similar to one of these, but made only for Sony or only for OEMs, meaning you might not be able to get the right drivers on the ASUS web site.
If none of those drivers work and those Yamaha drivers didn't work, either, then I honestly don't know anymore. This is just another example of why you shouldn't buy OEM desktops, especially not from tier-2 manufacturers like Sony.