Won't play Dolby 5.1 sound from HD-DVDs, only standard DVDs

Hi. I've got a A-30 hooked up to a Sony Ht-CT100 system via HDMI. The player had the original 1.0 firmware, but the problem I describe below has persisted even after updating the firmware to 3.0. HD-DVDs produce no sound whatsoever, even though when the disk starts playing, the LCD display on the Sony briefly reads "Dolby 5.1". But when I insert a standard-def DVD, the same "Dolby 5.1" notice pops up on the Sony, and the sound plays perfectly.

The only way I can get sound with HD-DVDs is to change the Audio setting on the Toshiba from "Auto HDMI" to "Downmixed PCM." (The other audio choice---labelled just "PCM"---also fails to produce any sound with HD-DVDs). I'm not happy with this as a long term solution, since it does seem to downgrade the positional quality to the sound effects.

Seems to me that Dolby 5.1 encoding should be the same with HD-DVDs and standard def DVDs. Not sure whether the problem is with Toshiba or Sony on this one. Any ideas? Thanks in advance---