On PEBs and power, I have the necessary data to do a 3- or 4-slot mini-PEB based on one that the Hunter Valley User Group did. The board is pretty straightforward and already has the flex-cable logic on it, so it would be another way to get expansion possibilities in user hands. I haven't looked at what they were using as a power supply yet (or the little board they put on the TI end of the expansion cable). It uses standard PEB cards too.
On cards that send audio to the console, there are several: The CorComp Triple Tech, The Berlin Speech Card (from Winfried Winkler, about 20 of these were made), the SNUG SPVMC, The Rave Speech Adapter, Marc's SID Master, the TI FORTI card, and the TI Video Controller. Greg's Speech Adapter will soon be on that list as well. I have all of these boards except for the Video Controller and the Triple Tech.