"no Audio Device Installed" After Installing Amd 5450 On N40L

Hi.  I have just installed a SAPPHIRE AMD Radeon 5450 graphics card in my N40L, running Windows Home Server 2011 SP1.  I'm getting video over HDMI but no audio.  Control Panel sound says "No audio device is installed", but Device Manager shows "AMD Hgh Definition Audio Device" under "Sound, video and game controllers".  Correct driver seems to be installed -- even used the AMD driver updater tool to update them.  Also, I've stared the Windows Audio Service.

Been battling this one for days, so any help greatly appreciated!

Thanks,
Matt