Currently, there is no way to blend music and microphone output and publish it using OpenTok, since OpenTok only receives input for publishing from the mic or a webcam. One thing you could do is play the uploaded file on the site as an audio file, and everyone will be able to hear the file along with the microphone input from OpenTok's stream.
To publish audio only using OpenTok, you can set publishVideo
to false when you initialize the Publisher. Documentation here.