Few weeks ago I've made a home-built low-noise stereo mike for my HMZ1 and there is a noticeable difference between the internal mike and the external one. I always thought that internal mike is quite decent before trying external one. :-) The external mike was made with standard 3.5mm stereo plug.

The files can be downloaded here:
https://soundcloud.com/crunchy_3d

The new mike has been tried on JVC and on Sony D50 sound recorder.