Some friends and I have started a weekly podcast, and have decided to add video to it, currently we are using Audacity to record audio and an SX30IS powershot to record video. When I place both of the files in my editing program and sync the audio track to the video, the audio and video are sync perfectly at the beginning but are off by ~4secs at the end of the 20 min video, any idea what is causing this and how to fix it?
Yes. I can tell you exactly what is wrong.
Your synch is off because the frame rate you 'think' you are recording, isn't the frame rate you are editing at. Consequently when you lay in the audio, it loses synch.
Specs of this camera say that it will record at '30fps' but it is highly likely you are actually shooting at a non-integer frame rate of 29.97fps, consequently you need to adjust the frame rate in the timeline to suit. This will more than likely be the case if you have the video mode set to 'NTSC' anywhere in the camera.
It might also be worth confirming the real frame rate using Quicktime player and checking in the movie inspector.
Pull-up/pull-down issues are almost always caused by a frame-rate imbalance.
If you are unable to adjust the frame-rate to correct this, you can use a pull-up/pull-down timeshift to fix the issue. If the issue is simply an integer to non-integer frame rate issue, a timeshift of 0.1% should do it.