Simple answer: you need an AC3 filter. An easy one to get is available at:
If you already have an AC3 filter but are still getting the same error, then it may not be installed correctly or you may need to install the latest version
Don't be surprised if your AC3 streams contained in .avi files have audio sync problems, though. For AC3, Ogm or Matroska containers should perform better.
Also, we suggest to download the latest stable version for best results.