Created attachment 1690 [details]
example remediated vxml-synthesis-extension.xsd

when using the vxml 2.1 xsd located at https://www.w3.org/TR/voicexml21/vxml.xsd to validate written VXML using an intelligent IDE(using PHPStorm from Jetbrains in my example) I receive an error for the following chunk of VXML:

<prompt>
        <audio src="http://sanitizedhost/voice/sanitizedfolderpath/mainmenu.wav">Welcome to Foo, how may I assist you? </audio>
      </prompt>

The advice I receive is that audio tag is not a valid subordinate tag of prompt. When I went digging through the xsd collection that make up the validation I found that vxml-synthesis-extension had an error notice in it, this error notice indicated that it had no concept of what executable.content was. I researched what that group was and it is the same group that appears to be missing in the vxml.speak complex type. 

To remedy I believe that the vxml-synthesis-extension.xsd needs the following remediations:
* <xsd:include schemaLocation="vxml.xsd"/> needs to be added such that this document is aware of what executable.content is.
* <xsd:group ref="executable.content"/> needs to be added into the vxml.speak complexType to permit Audio(among other subordinate tags that are part of the VXML 2.1 Specification) to be considered valid.

I will attach my copy of the xsd that I used to remediate the error in my IDE as an example.

Thank you for your time.