I can't really conceive of getting a divx without sound, if there was sound on the original file.
If you are missing a codec during conversion, divx will tell you and refuse to convert the file.
If the original file simply didn't have a soundtrack, divx would convert the file, but not if it was a codec issue.
If the file was corrupt, it shouldn't play at all.
It is somewhat possible, since you are talking about ripping from DVD recordings, that your friend was using Dolby sound codec, and you have the divx maintaining the AC3 codec for dolby sound, but you don't have that codec on your current machine. I'm not even absolutely certain this could happen, how would he be recording off the TV in Dolby? But theoretically that would do it.
Do have the K-Lite codec pack or similar installed on your current PC? What OS are you using? With 64 bit OS a lot of 32 bit codecs won't register properly.
There's actually a very simple reason that I didn't get sound (and the most likely) - I tripped the line input cord from the dvd player to the pc sound card. The sound doesn't get modified or anything, until I turn the mpg file into a divx - and if there was no sound on the mpg, there won't be on the divx.