Here's how I did it...
Part 1 - VMPro. I have a "queued" sequence that callers hear when in queue (duh). At the end of that sequence I have a "Post Dial" action. On the "specific" tab, I have "post dial wav" radio button ticked. I have navigated to the appropriate wav file in the appropriate box. And I have it sent to extension 98 (ext 98 is particular to my system)
Part 2 - IP Office. This was the tricky part. Because I wanted the wav file to be played over an OVERHEAD paging system, I needed to connect an interface from a loop-start CO postion to the paging amplifier. This was required because the IP Office required that the page be "answered" before it would play the wav file. Anyway, I connected amp to CO position, then put the CO position into a unique trunk group. Programmed LCR such that if a user dialed 9 + 8, it would use this particular trunk. For whatever reason, the IPOffice wouldn't work if I just used a standard shortcode for this process.
So, that's the story. It's really quite slick and works like a champ. Good luck.
Mike