I wanted Windows to send output to two devices. Same content. Just send it to my amplifier (via HDMI) and to a wireless audio transmitter. Both the HDMI and the analog audio devices are on the Acer Revo motherboard.
Windows 7 insisted that it could send output to one or the other, but not to both at once. I Googled and Googled, and I found others who wanted to do the same thing, but no one succeeding.
So I dropped $40 on a SIIG USB SoundWave 7.1 Digital audio device. It is just a USB audio ‘card’ for Windows, with speaker, headphone, and SPDIF outputs.
Since my amp has SPDIF input, I send the digital audio to the amp via SPDIF, and I used the headphone output to send the output to the wireless audio transmitter. The SIIG driver lets you turn on/off any of its outputs, so I was able to tell WINDOWS to send output to the SIIG (only), making Windows happy with its one-and-only-one-output-at-a-time policy, and I was able to use the SIIG to split the output to my two devices.