I've updated my T23 motherboard thread: viewtopic.php?f=28&t=37956
with the reference for the audio processor (AN17000A). May not be any help, but you could look in that area and see if there's any obvious damage. Remove the memory access panel, pull the module next to the front edge of the system and U22 is right there in the corner. The volume control pin is on pin 39. Unfortunately that puts it on the side of the IC closest to the front edge. Volume appears to be controlled by varying the voltage level on that pin. How that voltage is controlled by the two pushbuttons on the keyboard is still unknown to me. Most likely there's another controller which converts the pulses from the switches into an analog voltage that gets applied to that control pin.
Edit: Looks like ThinkWiki has some information... http://www.thinkwiki.org/wiki/Embedded_Controller_Chips
says that there's an H8S microcontroller on the T23 motherboard. I can confirm that as it's the larger of the two square chips underneath the "front" memory module. It has (had) a white label with an FRU of 30L2413 printed on it. ThinkWiki also says that the H8S "... controls various low-bandwidth devices such as keyboard, mouse, ...". So maybe it will pay to look around this chip as well.
Coincidentally, when I took a closer look at this "does not turn off" labeled T23 motherboard, I noticed that a chip resistor was sitting at a weird angle next to the H8S. It apparently has come loose from somewhere around in that area. Now I'm wondering if the problem I saw with this motherboard (it would not power off with the keyboard's button), now the donor of some inductors and serving as a test subject for circuit tracing, was due to this missing resistor