Need help - how to create a collar. web app for music making

I want to create an online web app that allows two users to collaborate on making music. So, If I was an artist from CA and wanted to work with an NYC artist, I would go on the app, upload an instrumental, record, save, and send. Then they would be able to play, record their part , and save. How can I do this? (functions, methods, languages needed, etc.)

Please ASAP! Have less than 4 days to create.