If you know how to be safe around high voltage stuff you could just open your monitor and break out the signal that goes to the internal speaker, it's a simple detatchable 2-pin connector (at least on the SC1224). I made a simple audio-out jack on my monitor that way: http://oi39.tinypic.com/5vt7yp.jpg
I would go for this solution. ofcourse if you don't want the stereo you can leave that out and connect everything straight to the audio out. To me thats a better and easier solution than breaking cables or even the monitor to get a signal out of it. http://rdemming.home.xs4all.nl/Atari/index.htm and look for the stereo upgrade link.