Estimado Francisco,
As one of the top media-developers on Freelancer, I'd like to hear some more about your project.
I can definitely create an audio-extraction script in PHP for YouTube videos, and output the audio in a chosen format (mp3/ogg/m4a) which I can add a drop-down for along with the form.
Do you have your own VPS to run the software on?
Let's discuss the details, and hope to hear from you,
Sebastiaan