I want to listen to audio while plasma is off with Onkyo AVR using HDMI

I just bought a new TX-SR705, and I have a question. I have a DVD player and a cable box connected to the receiver via HDMI, and then an HDMI cable out to my Panasonic plasma. When I'm listening to a cable TV music channel, or when I'm listening to a CD or SACD on the DVD player, I often like to switch off the plasma, since I'm listening to music. When I turn off the TV, the music stops playing! How can I listen to a source while the TV is off? I've read through the manual and cannot find any help there. Thanks, and I look forward to your reply.