Notebook validation fails when using IPython.display.Audio

When using IPython.display.Audio to listen to audio in Jupyter notebooks, the notebook validation fails. I am not sure whether this is a Jupyter or IPython.display.Audio bug.

I get two different errors:

`Notebook Validation failed: Additional properties are not allowed (u'execution_count' was unexpected):
{
"output_type": "display_data",
"data": {
"image/png": "iVBOR...z73wM=\" type=\"audio/wav\" />\n Your browser does not support the audio element.\n \n "
},
"execution_count": 153,
"metadata": {}
}`

`Notebook Validation failed: u'execution_count' is a required property:
{
"output_type": "execute_result",
"data": {
"text/html": "\n \n "
},
"metadata": {}
}`

I am using Jupyter 4.2.1 and IPython 5.1.0.

In addition, saving the notebook produces

Start by reproducing notebook validation with IPython.display.Audio under Jupyter 4.2.1 and IPython 5.1.0, then determine whether the generated output or the validator causes the execution_count errors. Done means Audio-containing notebooks validate successfully and saving them no longer produces the reported failure.

Written by the indexing model from the issue text.