When I run sox directly from the command line as ;

```
sox -r 8000 -c 1 <wavfilename> <mp3filename>

```

I get the following error message:

> 
> sox: Unknown output file format for
>  '747a.mp3': File type 'mp3' is not
>  known
> 
> 
> 

My machine is running the CentOS operating system.