Avi Joining help needed

[Nikon]

HI guys
I have a m8 who has probs joining dvd rips that is downloaded from the net,
The prob is when they are converted back to put back to cd the sound is always out and we cant find a way round it, This prob only occurs when we try joining say Windtalkers dvd rip and all other 2 cd version avi rips.
Can you guys reccommend a program for joining them properly the actual movie joins great but each time it is the sound that is out
Thx