There are several things. First you need to know if the xViD contains an AC3 codec for the audio. AC3 is the 5.1 channel audio. If it does contain AC3 then you would need to extract the audio first as a wav file and then use that audio file in Tmpeg. You can extract the wav file using virtualdub. Why dont u save urself all the trouble and just use Nero Vision express

It will do the complete job for you. Make sure you have the necessary plugins for Nero tho.