Comment 3 for bug 321028

It seems like the "help" is always 180 degrees backwards for Avidemux on Pulse Audio systems. At least it always is for my systems. Did you try the following:

In Avidemux do Edit -> Preferences -> Audio (and change the Audio Output to Pulse Audio). It is originally ALSA, and I know most instructions are backwards, but this always works for me. It should make your "error initializing device" go away. If not, play in that dialogue (just know how to get back to the original settings).

Now for the encoding options: You may need to use Synaptic, enable the multiverse repositories and download all the gstreamer libraries (good, bad and ugly) depending on what your are actually trying to do. You might also get the gstreamer alsa and ffmpeg packages as well.