This is very odd....
Is there any sort of rule in Outlook that perma deletes messages with .wav files (it's a stretch, but so is this issue
I would be surprised if it was anything to do with permissions if you are able to receive the email and see the attachment on the device....but stranger things have happened btwn Cisco/Unity/AD.
Sorry I can't be of more help. Try searching the Cisco forums, too....there may be something out there.