I'm trying to run HDMI to a projector (via HDMI-DVI adapter) for video, and Composite (RCA Red & White by using the headphone jack and a splitter cable) to an older theater-in-a-box type receiver for audio. But when I plug in the HDMI cable, the headphone jack shuts off - no sound. I've seen people describe ways to resolve this for older versions of Windows, but I have Windows 8 and I'm having trouble finding it.

From your description I understand that you want use HDMI and Headphone Jack Simultaneously.

As per the design you would not able to hear sound from both speakers and HDMI for a single application at one time.

Generally, in a normal circumstance we can only use one device as the output device. Example, the speakers sound is muted when the earphone is plugged to the computer.

This might be possible if you can have two different sound cards

Still you can try these options.

Follow these steps.

• Press Windows key + X.

• Now click control panel

• Open Sound panel

• Select Speakers as the default playback device

• Go to the "Recording" tab

• Right click and enable "Show Disabled Devices"

• A recording device called "Wave Out Mix", "Mono Mix" or "Stereo Mix" (this was my case) should appear

• Right click on the new device and click "Enable"

• Right click on the new device and click "Set as Default Device"

• Double click on the new device to open the Properties window

• Go to the "Listen" tab

• Click on the "Listen to this device" checkbox

• Select your HDMI device from the "Playback through this device" list