I'm about to launch a service where one of the feature is to upload files with an 'upload' button on a website. Some years ago, I made some program for iPhone, and I remember that it was impossible to upload an MP3 from the library, because each app is in its sandbox, though I was able to upload MP3 placed in the sandbox itself.
There is an old post on SO about the impossibility to upload from the library to a website:
A html5 web app for mobile safari to upload images from the Photos.app?
Is possible as of may 2012 for an iPhone/iPad to be prompted into the music library when clicking on an html upload button?