That'll be because that board isn't supported by Win 7, it's to old - have a read here: http://downloadcenter.intel.com/Detail_Desc.aspx?agr=Y&DwnldID=18246&ProdId=2613&lang=eng&OSVersion=Windows%207*&DownloadType=Drivers
You MAY have some luck attempting to use the Vista audio drivers for it under Win 7 or maybe even the Windows 7 IDT Audio drivers for the DG45ID if they'll install on your board. It appears you have a similar audio chip to that used on the DG45ID so you have nothing to loose by trying that. Unfortunately IDT don't release 'generic' drivers publically for their audio chips so it'll be down to you to try and come up with an unofficial solution.
Let us know how you get on!