Is this the forum you meant to ask this on?
But two things:
you're trying to use the output audio before you've closed the write handle so maybe the file isn't fully written yet
you run "start output.mp3" but will that find "/tmp/output.mp3" which is what you wrote? I suspect the working directory is not currently /tmp. So try "start /tmp/output.mp3" as well?
Or find a Python library which plays MP3s and use that.
And definitely check that the output.mp3 does get written and can be played with a normal MP3 player.