There are soooo many apps that convert audio files to MP3 and a whole lot more that are FREE, easy to use, and even produce higher quality output. Just check out any of the apps in the Similar Software section on this page or search MacUpdate for 'audio converter' or something similar.
It's shameful to charge $4 for this LAME (pardon the pun), one trick pony app when so many much better and more feature-rich free alternatives exist.
BTW, MP3 is an anachronistic, halfassed, low quality audio codec. The codec created specifically to replace MP3 is called AAC (Advanced Audio Codec), and it produces substantially higher fidelity output at lower nitrates. Give it a try and join the 21st century.