Sound is the first thing people notice when they find something irritating about a video.
Although video cameras have built-in microphones, try to use an external microphone whenever possible and connect it to the camera's mic jack.
If you find there is too much echo, try to use cloth to dampen the sound. Record your footage with subjects sitting on cloth chairs or move the subjects closer to drapes or wall-hangings. Hanging blankets out of sight of the camera, especially behind the camera, can help dampen stray noise.
While interviewing, leave a bit of ambient noise before and after questions. These quiet intervals can be helpful when editing the footage.
Avoid taping an entire session only to find out in the editing process that the audio was not recorded well. At the start of your shoot, plug some headphones into your camera and listen to make sure the audio is how you want it to be.
Vimeo Video School has great tutorials on capturing good sound that are worth taking a look: