Most of the time, Avidemux computes the offset and length of audio by doing size=time*bitrate. That works for all CBR streams.
For VBR, it does not work as bitrate is not constant.
To compute the start offset/size, Avidemux has to know how the bitrate changes. For that use Audio→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 libmad 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.
If you don't do it, 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.
When saving a dual audio AVI, the second audio stream cannot be VBR.