Micro Audio Converter
Developed by Aaron Smith, MicroAC is a nice GUI for
afconvert. In case you've never heard of
afconvert, it's a command line tool that Apple describes as a utility to "convert a source audio file to a new audio file with the specified file and data types". Rather than forcing you to set parameters by hand in Terminal, MicroAC provides an elegant and simple interface that makes it easy to convert audio files from one format to another.
MicroAC can convert "Core Audio compatible formats" such as 3GP Audio, 3GP-2 Audio, MPEG-4 (both for audio and Apple audiobooks), AIFF, WAVE and CAF. The app lets you drag & drop files to convert into the main window, and you can choose types, data formats, extensions, channels, rates, and destination in the lower section of the app. In actual usage, I was able to convert MP3 files to MP4 in less than 2 seconds.