Currently the visibility status of widgets like volumeWidget is not preserved upon restart of Avidemux. At the same time Avidemux uses PulseAudio Simple API (https://freedesktop.org/software/pulseaudio/doxygen/simple.html), which doesn't allow volume control, thus neigher the volume slider nor the mute toggle in Avidemux has any effect on the vast majority of Linux desktops. Of course, this is not a deal breaker but just a minor usability glitch (puzzling users by exposing a non-functional control), but it is aggravated by the lack of a method to hide a "broken" control permanently. Maybe it would be doable to implement QMainWindow::saveState in time for the next release?
(This topic might belong into the mass grave (http://avidemux.org/smif/index.php/board,7.0.html))