I opened a .vob file with Avidemux and made several deletions.
Then I saved the file, using the defaults on the opening page of Avidemux.
That is, I left the Video option as "Copy", the Audio option as "Copy", and the Format as "AVI".
I was careful when saving the file to include the extension .avi because I read somewhere that Avidemux does not automatically give saved files extensions.
The resultant file had no sound. The video was fine but audio track was missing.
When I tried saving it as an .mpg file, as an alternative to a .avi file, it again played without sound.
I tried changing the audio to .mp3 but to no avail.
I scanned the forums and discussion pages and discovered that a poster had the same problem and he solved it by changing the Format to MPEG-PS(A+V), leaving the other options without change, and using the extension .mpg when saving the file.
I tried this and it worked. Appended below is the screen with the options I used (thanks to the poster) to overcome the loss of audio when converting .vob to .mpg.