Works fine with a pair of Pioneer IEMs I have, as well as the bundled headphones. I also used the IEM that was bundled with the Nokia N97 Mini with no issues.

I'm guessing the issue is more with the N9 not talking the right language to the Mic/Volume portion of your Sony earbuds... similar to how the N97 bundled IEM with play/pause/mic/volume sound like tins when used on my Macbook Pro.

On my Macbook Pro, the "solution" (if you can call it that) is to not insert the stereo jack 100% into the socket... although you probably won't want to do this all the time :P