This issue is common with older OS such as Windows 7, if the video was made with a camcorder.
In the recent updates, there is no support for AC3 codec, but you can use VLC Media Player as a workaround, to convert your videos.
1) Download VLC Media Player: https://www.videolan.org/vlc/
2) Open your video in the player, and go to Media->Convert/Save menu:
3) Click on '+' to add your file, then click 'Convert/Stream' button in the bottom right corner:
4) In appeared option, select 'Profile' Video - H.264 + Mp3(mp4) and click on the settings icon:
5) Proceed to 'Audio Codec' and adjust 'Bitrate' to 196 or 320 Kb/s, and make sure that sample rate is 44100 Hz or 48000 Hz. This will ensure that the audio quality in the output will not be reduced.
6) Click 'Save', click 'Browse' next to 'Destination File', then start the process.
The resulted video can be opened in the program without any issues. If you experience issues with the converted video, please, contact us via Support Page.