Following a fresh install of Windows 7 (I haven't tried an in place upgrade as I just don't trust 'em !) the OS sees the Sigmatel device and reports it to be working, but the sound just doesn't work. Any attempt to download and install the latest Vista drivers also fails to cure the issue. The fix
Download the latest x32 or x64 (as appropriate for your computer) Vista driver from the Dell website and leave the EXE in a folder somewhere
Or get them direct from us, click for the 32 bit driver
or for the 64 bit driver
Right click computer, and choose manage, then device manager
In the Sound, Video and Game controller group you will see a device called "Windows High Defintion" - delete this by selecting and pressing delete
Now having removed this device from your system you should run the Dell Sigmatel Driver EXE you downloaded previously
Your system will prompt for a restart upon install
When you restart you will get the reassuring sound of the Windows 7 start up tune and all will now be well !
Hope this helps someone, took me an hour today to figure it out.