- Two cannel audio out can be split into a whisper speaker & output speaker pair.
Use software to control the left/right channels of a stereo audio output independently. Enforce different volume levels either in software or with hardware. Ideally you would want to use the built-in computer speaker, I think, for voice output. Thats probably a little more tricky.
- Audio in ( microphone ) can serve as switch input. If microphone input is stereo, then two switch inputs may be possible.
This modification will require some sort of power source. I suggest using the USB ports for a small amount of power. The microphone inputs can be pulled low when the switch is open, and driven high when the switch is closed. This will create reliable switch input.
- A full-screen application running on the tablet PC touchscreen can fully duplicate dynavox system functionality. Full access to PC capabilities increases the flexibility of the device. Objects such as a USB infrared control module and additional microphone inputs could be added. I assume by default that the thing will run linux, although getting all the drivers working properly for linux on a tablet PC might be "fun"