Please Note: For information on how to do this in the latest DivX Converter please go here.
You have two options for maintaining your original (AC3, MP3) audio track with converter.
Available only in Home Theater, HD 720p, and HD 1080p profiles
If you want to maintain the source audio for one particular session, make sure you check the "maintain source audio..." option under "Media Options" once you have dropped the file into converter.
If you would like to enable the "Maintain source audio" as a default (This would preserve the audio track of all inputted files, for all sessions), you would right click Converter and select preferences. From there you would check "Maintain source audio" under the "Default Audio Preferences"
* Remember it's only available in Home Theater, HD 720p, and HD 1080p profiles!