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?
Apple will not allow you to do this. Although it may be possible using private APIs or perhaps the Media Player framework, it will not be accepted by Apple.