From the CLI how so I select AAC as the audio codec?
Command line possible arguments :
--audio-codec, set audio codec (MP2/MP3/AC3/NONE (WAV PCM)/TWOLAME/COPY) (one arg )
Think not setting codec, think setting encoder. Depending on Avidemux build configuration, you can choose between LavAAC, FDK_AAC and Faac as value for --audio-codec when encoding audio to AAC.
The documentation other than the actual source code is utterly outdated, unfortunately.