Camtasia 9 and later on Windows 8.1 and later no longer needs a 3rd party AC-3 audio filter since Camtasia uses the audio decoding provided by Windows
Camtasia Studio 8
The AVCHD format is described here. Camtasia Studio 8.4.0 supports decoding the MTS (or M2TS) container, H.264 video, and PCM audio. Support for the AC-3 audio format does not ship with Camtasia Studio. AC-3 support must be installed separately. Please note that using certain encoding/decoding technology and/or formats in some countries may require the payment of applicable license fees.
If the necessary software is already installed, you can import and use the file as you would any other video file. If not, the steps below explain how you can use these files with Camtasia Studio.
Install the software that came with your camera:
Your camera may have come with software that would allow you to play the video format produced by your camera. This may or may not install the codecs necessary for Camtasia Studio to decode and use the files from your camera. After installing this software, close and reopen Camtasia Studio, and then try to import a video from your camera.
Adjust the settings on your camera to use the PCM audio format:
PCM is uncompressed audio. This will result in larger video files, and will reduce the amount of time you are able to record on your device. But the audio will be high quality, and you will not require an AC-3 decoder in order to play the files from your camera.
Install 3rd party software to decode the AC-3 format:
Download and install the AC-3 Filter from here.
Use a 3rd party video conversion tool to convert the video into a format that will import into Camtasia Studio:
Details on supported formats, and recommendations for conversion tools can be found here.