Import settings and save.
(Output Format) is set to (MP4 Muxer), it will be saved.
(Audio) is set to (Outoput), it is not saved.
Why?
You have to manually edit defaultSettings.py.
Set your preferences, then close the program. Check lastEdit.py, and copy adm.audio* lines into defaultSettings.py
Before these line put
if adm.audioTotalTracksCount() > 0:
then indent the following adm.audio* lines.
Example:
if adm.audioTotalTracksCount() > 0:
adm.audioClearTracks()
adm.audioAddTrack(0)
adm.audioCodec(0, "LavAC3", "bitrate=160")
adm.audioSetMixer(0, "STEREO");
It will onyl work, if you do a "Load saved settings" (Ctrl+R) AFTER you loaded a video.
Audio encoder selected for the first audio track was (and still is) not saved as default configuration on purpose as Avidemux' audio configuration is tied to availability of an audio track (the application would crash if it is missing).
"Load saved settings" would help only if you have added commands to configure a particular audio encoder manually, knowing that your changes will be overwritten the next time you use "save settings as default" menu action. I would instead recommend to use the "custom" subdirectory of Avidemux profile for such purposes.