No audio - What am I doing wrong?

Hey, so I'm new to this program, so please be gentle, if I am making an obvious mistake.

My preferred genre is .MP4 and recently I have been trying to convert a number of .MKV files to MP4. This is what actually led me to start using DivX which didn't charge too much, in my opinion (unlike VideoProC) and didn't seem to suddenly struggle, to convert newer files (such as FormatFactory and 'Any Video Converter').

The price is obviously perfect and the quality of the conversion is always great...when what I want is formats, other than .MP4. Whenever I convert anything to MP4, suddenly I have lost the sound. I know it is not that the original file doesn't have sound, because (1) I can convert to other formats and get sound and (2) I can play the original and get sound, so I don't get it. I don't touch any of the other settings (which, as I say, is unnecessary, given any other format), so I am not sure what I am doing wrong or not doing right. Any thoughts?