Ah, I see. That must be why I don't have any option called "External Microphone" in the sources list. I can only use the internal one, and if I select analog line-in with the external mic plugged there, it doesn't work either. The recording is completely silent (the line input still works with an actual analog line cable plugged to the output another audio device).
So it seems I can't use external mics with my iRiver (unless I use an external preamp, but that adds extra bulk).
Thanks for clearing that up.