MotionApps just released their first voice recorder for Treo 600: mVoice 1.0. The features look very good to me, particularly one-button recording and use of an industry standard sound file format (though other voice recorders have those too).
There are now four voice recorders for Treo 600 that I'm aware of. All of them require the 1.12 version software/firmware upgrade, of course. They are:
You can search the TC forum for people's comments about each of these, of course. SoundRec is the only free one (donations are accepted).