Avidemux is GPL V2.
It contains other software included as source with the following licenses :
Name   : liba52
URL    : http://liba52.sf.net
License: GPL

Name    : Mad
URL     : http://www.underbit.com/products/mad
License : GPL

Name   : Spidermonky
URL    : https://developer.mozilla.org/en/SpiderMonkey
License: GPLv2

Name : Tinypy
URL: http://www.tinypy.org
License : MIT + PYTHON SOFTWARE FOUNDATION LICENSE VERSION 2 

Name   :  TwoLame
URL    :  http://twolame.sf.net
License:  LGPL

URL    : http://mp4v2.googlecode.com/svn
License: LGPL
Rev: 485
Name    : libass
URL     : https://github.com/libass/libass.git
License : GPL >=2
Version : commit 23de9995103c16ceceba19cae87e328be12fa8e4 / 3 August , slightly more recent than 0.12.3

Name    : Secret Rabit Code
URL     : http://www.mega-nerd.com/SRC
License : GPL 

URL    : https://github.com/saintdev/libmkv.git
License:  GPL
Rev: 0.6.5.1 / d2906c03cc8f91fca0074e02bb14504a0e85bce6
Name   : fglrx_info
URL    : Splitted-Desktop Systems
License: GPLv2

Name    : ffmpeg
URL     : http://www.ffmpeg.org
License : GPL >=2

Name    : libjson
URL     : https://sourceforge.net/projects/libjson/
License : GPL 

Name   : BVecor
URL    : http://berenger.eu/blog/
License: LGPL