Use Audacity To Convert Multiple Files To MP3

 In Audacity, select Tools->Macros.  That brings up the Manage Macros window.

In the list of macros, select MP3 Conversion.  

Down at the bottom, where it says "Apply macro to:", click the "Files..." button.

In the "Select files for batch processing..." window, select the files you wish to convert, and click the Open button.