I run a system having dual os ie Vista premium and Windows 7 ultimate.
Primarily it is an HP pavilion desktop.
So when i boot in vista, the realtek hda manager is enabled automatically on startup and icon is visible in notification area.
Whereas when i boot in win7 it doesn't start automatically.Every time i have to start it manually from control panel.
i had the same problem...when i installed the drivers for the soundcard after a clean install of windows 7 , the OS loaded drivers on its own. But when i installed the vista drivers for the card via manual install, the sound died out a while later while i was gaming. When i uninstalled it, after the reboot the sound was back to normal. Uninstall the drivers from Windows7 and allow the OS to detect and install the hardware.