It seems to me , Windows system do not care external or internal sound cards. It sends sound to all output devices,
Once you add a hardware, the Device Manger add the new hardware and whenever sound is to output, ALL devices will play. Unless your un-check one in Control Panel -Sound
Mine is Window 7 and All-in-one Motherboard with sound device and I use a USB external sound box , called Virtual 7.1 CH Sound box, so it will have enough and noiseless sound level for my headphones.
This is the same as Video. I normally use small VGA monitor (22") and when I watch want to ball games on the Web BT , I HDMI-plug to my LED TV and both videos are present .
Yes, your method should work though the setting is tedious. what I had above will only output all devices and cannot redirect music only to 1 output device. But my questuin is if the stereo system is playing, can you hear or bother to hear the PC speaker giving you the email alert signal ?