How to do mass renaming in Automator:
1.) In the Library click "Finder" to limit yourself to the Finder actions.
2.) Drag the "Get Specified Finder Items" into the workflow area. Here you specify the items we will work on, wich you can either do by dragging them into the action or by pressing the + symbol and selecting them.
3.) Drag the "Rename Finder Items" action into the workflow area. We don't want to work on copies in this case, so answer the warning with "Don't add" (a copy action).
4.) Change the first pop-up menu to "Add text" and type ".mp3" into the "Add this text" field and keep the pop-up menu to the right of that field at "after name".
Press the "Run" button in the toolbar. Be done.