Onebox play button greyed out on attachment

Greetings friendly friends

I’m working with a voicemail forwarder that emails the .wav file to our community. I’m having some problems with the onebox: The file gets attached as .wav, so it is embedded in the post, and clicking allows download.
Copying the link into a new post creates a onebox as expected, but the play button immediately greys out. I presume this is a forwarder problem.
Is there any way to automatically display a player/onebox for certain file extensions attached to a post? Much simpler to display a play button than expect users to all download the file, play it, then come back to the browser.