AFAIK the A4000 had a header for mixing CD audio.
I haven't looked at an A1200 motherboard in a while, but there is an audio input on the trapdoor slot. 146=audio gnd, 147=audio left, 148=audio right.
You might be able to trace them to a point where you can solder something in. Alternatively just run the audio out to another set of connectors and use an external mixer.
