It is sound advice to always use a microphone when you are recording talking head / interview type videos. Although this will give you much better quality audio than just using the onboard mic on the camera, it can results in you getting good quality audio in just one ear (this is normally caused by using a mono mic with a stereo camera). While some more advanced video editing applications will be able to sort this out for you, you might need to use a dedicated audio editing program to get the good quality audio in both ears. The video and instructions below demonstrate how to do this fairly simply using Camtasia as your video editing application and the free, open source Audacity audio editor. The process would work in a similar way with other entry level editing applications such as Windows MovieMaker, though as the video demonstrates you might have to first convert the audio track to a format that Audacity likes, using something like WinFF to convert your audio to a compatible file format.
Step by step guide: audioBothChannelsGuide