Play sound on Twilio call when button is clicked (WebRTC & Twilio)

Twilio provide a javascript client sample: [login to view URL] Note: It would help if you set this up yourself to understand what I am trying to achieve By default this allows you to call a number, and speak via the microphone. (This works!) I want to add a button to the page, that when clicked will play some audio on the call (e.g. mp3/wav) that both caller & receiver will hear. You will need WebRTC experience to complete this task. I believe you have to add a media track to the stream/peerConnection. If you have this experience please ensure to include 'WebRTC' in your response to this project otherwise I will ignore your response as you haven't read the description.