Thanks for your inquiry. I am afraid setting default player for mp3 is more related to your OS instead PDF file. You may set some default mp3 player in your OS for the purpose. Please share some more details if I could not understand your requirements.
Thanks for sharing the sample document, After initial investigation, we have logged a ticket PDFNEWNET-39495 for further investigation and resolution. We will keep you update about the issue resolution progress within this forum thread.
Thanks for your inquiry. I am afraid your above reported issue is still not resolved as product team is busy in resolving other issues in the queue, reported earlier. We will notify you as soon as we made some significant progress towards issues resolution.