I use a free program called "Any Video Converter" which allows you to convert Media from pretty much any format (MP4 to MP3s, AVI to Webms, etc). I am not aware of any programs that can do it on a mobile device as it requires a bit of computing power so you may need a PC.
Don't know about an iPhone, but it's so easy on a pc. I use Audition, but that costs. However I'm pretty sure that it would be just as easy in Audacity, which is free and it would also give you the chance to edit out any bits you didn't want in the mp3.