I have a similar problem in the same model. I need the Avi so i can edit the video. 
  
I do the backup directly from the DVR to a USB in AVI format but it doesn't have the audio. I can play it and i only see the video, I'm pretty sure is not about the codecs 
The backups  in .sec and the .DVR formats however have the audio Ok. 
  
When i do the backup using the smartviewer installed in my PC the result is always SEC format with the SAMSUNG player 
  
Any help is aprecciated.