Hi Oli, I was looking over the A1200 CPU slot and noticed that it Has Left and Right Audio signals on it... A quick hunt around some technical documents shows that any signals piped into these pins will be mixed with the Native Amiga Audio and squerted out of the Audio RCA's at the back of the Amiga.
If you include a sound chip on the board (I seem to remember you saing somthing to that effect), then could you pump the Audio output via those pins and make one really cool Amiga accelerator and soundcard in one!!!! :-)