VBR MP3

1.Background

Most of the time, avidemux computes the offset and length of audio by doing size= time * bitrate. That works for all CBR stream.

For VBR it does not work as bitrate is not constant.

2.Audio time map

Audio time map 

To compute the start offset/size, avidemux has to know how the bitrate changes. For that use Audio source->Build vbr time map.

Avidemux will then decompress the whole stream one time and build a time map to know how to handle the stream. You will need Mad installed, else it will do nothing.

When the time map is built, avidemux will have a conversion table between time and offset, and you will be able to split or save the avi in a VBR way

And if i don't ? 

You will end up with a badly desynced (potentially with too short audio) avi. Even if you just load and save it.

You MUST do it when dealing with VBR mp3. Period.

The second audio stream for avi cannot be VBR.