Hi.
Installed flatpak (flathub.org) version of Avidemux on Fedora 30 Cinnamon.
AAC (FDK) is not listed in "Audio Output" dropdown list.
Just tried to download the appimage variant of Avidemux, there I found AAC (FDK) as an option - so it seems to only affect the flatpak variant.
The right place to report packaging issues or submit packaging RFE for third-party Avidemux packages is the respective third party, in this case https://github.com/flathub/org.avidemux.Avidemux/issues
For best experience most of the time, build Avidemux from source, which is trivial on Linux: https://github.com/mean00/avidemux2/blob/master/createRpmFromSourceFedora.bash
You even don't need to package the application: https://github.com/mean00/avidemux2/blob/master/run_avidemux_template.sh