text: Title: JSON.parse error in examples/server
  Body: I am trying out this example: https://github.com/jlaine/aiortc/tree/master/examples/server 
  
  I can get to the server. But when I try to start either audio/video I get the following error:
  **SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data**
  
  I am seeing this error on my system (Debian), I suspect it might have something to do with that:
  **av.AVError: [Errno 1094995529] Invalid data found when processing input: 'demo-instruct.wav'**
  
  Extra remarks:
  - demo_instruct.wav is present
  
  Can somebody help me further with this?