Yes, I updated my CMI8738 drivers via windows update. Why C-MEDIA web has older content is a mystery to me...

Windows Update drivers, while often having newer release dates than drivers from the hardware manufacturer, are often older versions because they have been in testing at Microsoft's Windows Hardware Quality Labs (WHQL) for certification. The hardware manufacturer (e.g. C-Media) has to submit a driver package to the WHQL labs, which then gets tested and certified (or not) for official use with Windows. The only real way to know is to check the driver file versions.

The only reason I see in actions of C-Media: they are lazy to update site. I'm sure they periodicaly release new version of driver for internal use and when question about WHQL arise they send latest available drivers to Microsoft. That's why WindowsUpdate have ver. 643, but C-Media site have ver. 639...

By the way I send e-mails to C-Media but they never answer.