Blob URLs fail as src attributes for Audio elements if the URLs are created without the second (optional) parameter.


## Expected behavior:

Given I have valid stream data created by MediaRecorder
When I create a new Blob
    AND I create a Blob URL
    AND I use the URL as the src attribute for an Audio element with the controls attribute
Then I can play (and hear) the recorded audio

## Actual behavior:

Given I have valid stream data created by MediaRecorder
When I create a new Blob
    AND I create a Blob URL
    AND I use the URL as the src attribute for an Audio element with the controls attribute
Then I can't play the recorded audio
    AND I _can_ see the word "Error" on the Audio element

I've provided [a working example in this repository](https://github.com/tedsecretsource/sound-test) and provided more information on problem and the workaround in the READMEs (one at the root level and one in the src/tests folder).

Basically, the bottom line is, at present the second parameter (options) of `new Blob(stream, {type: 'audio/mp4'})` is required. Leaving it blank should work because it is marked as optional in the MDN documentation and a functional default should be provided. At present, leaving it blank can and does result in the Actual Behavior described above.