See this documentation for the salt:prompt element:
http://msdn.microsoft.com/en-us/library/ms871806.aspx, specifically option 2:
-
Referenced prompt. Another method to generate prompt speech is to use an external source file. The source file can be either a text-only file or an audio file. If the source file contains only text, the entire file content is spoken to the user. An audio file, however, is played for the user. The path to a referenced path can be relative to the current directory or an absolute path to the file. The prompts can exist on a different server, but as long as a network connection is established, the source file can be accessed. The following example opens a text file named welcome.ssml.
<salt:prompt id="Prompt1">
<salt:content type="application/ssml+xml" href="welcome.ssml" />
</salt:prompt>
The following example uses a .wav audio file. The behavior of the page is identical to that of the preceding example except that the referenced prompt is an audio file.
<salt:prompt id="Prompt1">
<salt:content type="audio/wav" href="http://localhost/hello.wav" />
</salt:prompt>
I.e. change your SALT app to use <
salt:content type="audio/wav" href="http://blahbah/blah/greeting.wav"/> instead of the current <audio> element/
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