What that error message means is that the device drivers have not been submitted to WHQL (Windows Hardware Quality Labs) for testing and signing.
In order for drivers to be signed, they need to pass certain WHQL quality tests. This does NOT mean that the drivers are bug free.
Vendors can choose whether or not they want to submit drivers for signing.
You can ignore this warning and install the driver. It's almost a scare tactic to get vendors to get their drivers signed. In general, signed drivers are a good thing, since you're assured at least some degree of quality in the drivers, but signing is not the end all and be all of driver quality. If the vendor chooses not to get the drivers signed, you have no choice but to use unsigned drivers.
In the grand scheme of things, unsigned drivers are not that big of a deal. Install the unsigned drivers and enjoy your sound card.