I have a Canon 6D DSLR. It has a monaural microphone input, and a headphone out. So far the audio I've done with the internal mic have been terrible, including either AF noises, image stabilizer ...
The Nikon d3100 does not have an external microphone port, which is normally not a problem (built in mic is great) unless your doing AF-A, then you get lens noises. For a shot that requires zooming ...