Unless there are multiple hdmi outputs from the sources, then you cannot. The home theater has now become the switching device.
What you want to do is hook the sources straight to the TV (DVD and Cable Box), and use an audio out from the TV to the home theater. You will likely have a digital audio out from the TV. This will carry the digital audio (as does the hdmi).
Find the corresponding digital audio input on the home theater. Now whatever (digital) source you watch on the tv will send that audio signal to the home theater. Use just the TV and it's internal audio, or use the home theater if you like. A Guy