The switches behind the LCD screen do all the audio switching... nothing is automatic. However, if you're using external mics on channels 1 and 2, the internal mic is recorded to channels 3 and 4 (in all but DV25).
The HVX records sound to the P2 cards whenever it records video to P2 cards... there's lots of discussion of how good this is on the DVX; not much said yet on the HVX. The DVX was extremely good for a video camera, not as good as decent external equipment...
so i'll be using one external mic and that's it. so i plug it in, turn it 'on' in the switches behind the LCD screen, and make sure all other mics are turned off, right?
1. Plug it in
2. set one (or both) of the inputs behind the LCD to the input you plugged it into (e.g., set CH 1 to Input 1)
3. turn on phantom power (+48V) if the mic needs it, and
4. set the appropriate switch on the front of the camera to "MIC" rather than "LINE".
Simple, huh? It's those sorta-hidden mic/line switches that are often at the root of problems...