What about just using iTunes? In the preferences, set your import settings up to use the MP3 encoder and choose your bitrate settings. Then select your whole library (Cmd+A should do it) and right-click and choose "create MP3 version". It'll use those bitrate settings and get it going.
You can also choose to show the column called "Kind" and sort by it. Then you can just select the files that aren't already MP3 files to make it quicker.
I do not understand what I do. For what I want to do, I do not do. But what I hate, I do.