I'm not sure what is causing it, but I have had a similar issue before with Dolby Digital output from my Scientific Atlanta cable box. On probably two occasions I've found that the cable box quits providing digital audio output when set to DD. Instead, it only works when set to PCM. A few days later, turning Dolby Digital back on would work fine. Since your HDTV's EDID probably tells the cable box that it wants PCM rather than Dolby Digital, it probably is causing the cable box to decode everything to PCM when the digital output is set to HDMI.