When I first started on this journey of converting aiff files to mp3s almost every converter applications that I tried had some sort of extra programs that would take over your browser.
First off you will need to download WinFF. WinFF is an Opensource absolutely free application with NO spyware, No Crapware etc in it. WinFF is a gui front end that also installs the command line program FFmpeg. FFmpeg is the program that will do all the heavy lifting.
First you need to click on add. select all the files you want to add to the conversion list. Make sure you set the output details to your needs.
When done hit the convert button and voila! Everything is converted!
Yes! it really is that easy to convert .aif to MP3 for free! Let me know if it worked for you.