Consumer camcorders have their limitations when it comes to audio. But there's a neat device that can give your camcorder some professional audio capabilities.
For starters, most consumer camcorders have one stereo audio track. Those camcorders that have a connection for an external microphone can usually accommodate only one. And the output from mono microphones (most external microphones you'll find are mono) is sent to only the left or right channel of the stereo track. This means if you want the voice of your interview subject to come out of both left and right speakers when you play your final video, you'll need to copy the voice from one side of the track to the other during your edit.
By contrast, professional cameras usually feature two separate microphone inputs, which comes in handy when you want to record a conversation between two people. Professional audio cables are sturdier than consumer audio cables. Pro cables are also better shielded, which helps to keep unwanted noise, like radio frequencies and electrical hum, from invading your interview. Professional audio cables also have sturdy, three-pin connectors, called XLR connectors. XLR connectors actually latch (that's what the "L" in "XLR" stands for) into the camera inputs. This means you can't accidentally pull out the audio cable during a recording - something that can happen with a consumer-style mini plug.
"Right," you say. "I'd love to use more professional audio gear. But my consumer budget just won't allow me to buy a professional camera and the gear associated with it. What do I do?"
The answer in two words: audio adapter.
An audio adapter contains two XLR inputs, line/mic switches and a mono/stereo selector. It also features two volume controls as well as a mini-jack where you can plug in a wireless microphone receiver. A short cable with a mini plug connects the output of the adapter to the microphone input of your camera.
The adapter mounts on your tripod. You then mount your camera on the adapter.
An adapter is a great, low-cost option to "soup up" your camcorder's audio capabilities. Companies like BeachTek make adapters for specific camera brands as well as "one size fits all" models. BeachTek currently advertises one such model, the DXA-2s Dual XLR Compact Adapter. B&H Photo Video also offers BeachTek products - so visit both companies to do some price comparisons. Each company also occasionally offers used units or floor models at discounted prices.
By the way, if you chose to go the adapter route you're also going to need professional audio cables. But if you can't afford professional microphones with XLR connectors as well, don't despair. You can continue to use your lapel microphone with the mini plug by purchasing an audio cable that has a female mini connector at one end and a professional male XLR connector on the other. You'll find such a cable made by a company called Comprehensive. A 25-footer should cost you around $20.