Hooking up Cisco Explorer 4642HDC through Sharp HT-SB600

Just bought a new soundbar which comes with a pseudo-receiver (Sharp HT-SB600). I have my computer and my Cisco 4642HDC routed through it with HDMI. The computer works fine, but the Cisco will not. At most I get about a second of sound with a black screen before even that kicks off. The TV appears to be attempting to receive the signal (a pop-up tells me that there's something 720p trying to come through).

The Cisco doesn't respond at all to controls even during the brief period that the sound is working (tried changing channels rapidly, it always stayed on the same station).

I've already verified that all the HDMI cables and ports are working through lots of swapping. The Cisco works fine when I plug it straight into the TV. Happens on HD and regular channels, and with 720 and 480.

Any thoughts on what I'm doing wrong here?