WordPress hook

Hello,

A customer of mine has running his website with WP3, which I’ve setup.
Now he came to me with a question I’m not sure how to solve.

He would like to upload mp3 files to a blogpost, nothing special about that, but, on the public view side there will be a flashplayer that will play that particular mp3 file.

That flashplayer is feeded by an XML file, the function that feeds the XML file is already finished and it’s working outside the scope of WP.

Now the thing that’s blocking me is how to trigger my custom function when he uploads a mp3 file?

Showing the flashplayer in a blog post isn’t an issue, the only issue that I have is the method to trigger my function that will feed the XML file.

Anyone can help me out here?

Thanks!