This issue is common with older OS such as Windows 7, if the video was made with a camcorder.
Also, in the recent update 24H2 for Windows 11, Microsoft has removed support for AC3 codec as well.
You may try installing this codec pack to bring back the support in your system:
1) Download the pack: Link
2) Right-click the downloaded file and select "Run as administrator".
3) Click Install next to both Decoder MFT and Encoder MFT:
4) After the installation is complete, please reboot your PC and try adding your files into Movavi software again.
Alternatively, 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 need further assistance, please, contact us via Help Center.