as a relatively new Avidemux user (I use 2.6.12 64-bits on Windows 7), I regularly perform re-encoding of H.264 mp4 files using AAC as sound, downloaded from a professional tv recorder (save.tv). Most of my activities is using the Mplayer Delogo filter (which is kind of hidden in Video-Filters-Sharpness rather than in Video-Filters-Miscellaneous, as some available documentation still says), to get rid of annoying TV station logos and temporary on-screen text which have nothing to do with the video in question.
Now, sometimes, close to the end, the TV broadcasts contain a couple of seconds nasty sound saying e.g. "And now we will be broadcasting this next very interesting item..." which I like to erase from the sound.
I have been looking for a possibility to mute a part of a video. Haven't found an option, but a little extra work will do the trick: Set the Audio Output to AAC (Faac), then Filters-Gain-Gain mode:manual(dB) and select -10.0 which is the lowest value available.
The result is not mute but lesser volume. So I produce the same process 4 times in a row, creating 4 files subsequently, and in the end (the 4th file) I have so low a volume as sound that it may be considered mute.
Before that I'd tried to disable the audio track in Audio-Select Track, which however lead to a crash when trying to append the file chunk w/o audio track to the one before which has audio.
I hope this may be helpful to someone out there.