If they're not "protected" (I use the term loosely), iTunes will do it for you. Go to Edit --> Preferences, to the Import Options and choose your format/bitrate. Then right-click the file in your iTunes library adn "Create MP3 version" will be on the menu.
You can even add Kind to the list of columns, sort by that column, shift-click to select all files of that kind and then right-click and you can batch-convert.
I do not understand what I do. For what I want to do, I do not do. But what I hate, I do.