Most likely that the issue refers to the codec.
There are 2 ways of correcting it:
1) By changing the output files settings:
- Please go to Output format preset Settings, 'Audio' tab.
- Select MP3 audio codec. Press Ok.
- Try to convert video file once again.
2) By installing codecs that enable your video to play correctly.
You can try the KLite Codec Pack, to get all popular media codecs to your computer.
You can also use a different video player that does not depend on codecs initially installed in your system.
Examples of such players are GOM player and VLC player.