Then you need to do audio separately.
Load the first of your new files, select Audio, full processing.
Select file, save .wav
Open the second file (not append), and do the same.
You will now have two audio files.
Load the first video again, select audio, no audio.
Append the second part.
Select audio, no audio.
Select video, direct stream copy.
Select file, save avi.
You now have a joined video.
Get Goldwave or audacity, open the first .wav
Join the second to it, and select file, save as .wav PCM signed 16 bit.
You now have a joined audio and a joined video.
To put them back together, open virtualdub, open your joined video.
Select audio, wav audio, and browse for the joined .wav