Avidemux doesn't save videos correctly.

[Fleabag]

One of the other interesting discoveries that I made is that the problem that I mentioned seems to happen when I have the "Gain" option of the Audio filters menu set to "Automatic" (I suspect that because I successfully saved a video file after making sure that the "Gain" option was set to "None").

[nibbles]

I took an xvid+mp3+avi, and I opened it in 2.5.6.  I selected an I-Frame.  I hit point A.  I selected a later I-Frame and set that as B.  I chose mpeg2enc+mp2(lav)+mpeg-ps like you did.  I changed the audio filter Gain Mode: to Automatic (max -3 dB).  I hit save and gave it name.vob.  It worked.  I can't reproduce your error in avidemux2_qt4.  You never pasted in the terminal output from when it failed.  You never opened the original video in MediInfo GUI and pasted in the output so we could see what you were starting with.  We can't accomplish anything if you can't make a reproducible test case, but I'm glad it's working for you.

[Fleabag]

I used an extension of.mpg instead of .vob which MAY account for the difference.

[Agent_007]

File extensions shouldn't matter (unless there is a media player, that is poorly coded).

[styrol]

File extensions shouldn't matter (unless there is a media player, that is poorly coded).

I don't know if it's (only) the proper coding. MPlayer and VLC are great (also) because they play many non-conform media files. QuickTime Player and mainly Windows Media Player are rather picky on the 'right' file extension.

For example Apple uses special file extension on MP4 files depending on features, e.g. "M4V" for MP4 files with AC-3 audio. AC-3 in MP4 container is not standardized by MPEG. QuickTime without Perian Plugin won't play such file if the extension is MP4.
Other example, try to play a file labeled as MP4 (a MP4 file streamed over rtmp and recorded by Flvstreamer), but which is actually a FLV file (Flvsteamer repacks the video and audio streams in a FLV container).  In practice file extension often matters if you don't use software like MPlayer and VLC (which come with their own decoders and splitters).