I was flashing with the new D.6 BIOS and I turned my Xbox off. I turned it back on (I was testing out stuff, nothing major), and I turned it off again. About 5 minutes later, I turned it on and I got no video/sound. Hmm, so I switch my S-Video selector to my other Xbox and booted it, and it worked fine. I switched back to the other non-working Xbox and still got no video and sound. I figured I will just turn off and then back on, and no video/sound. And, the light on the Xbox was still green. I figured I'll check the FTP connection, lights on my router were all on (good sign), but it couldn't make a connection.
So I put the Matrix back in mode 3 and vialo! It worked with video and sound.
Any ideas?