Edit: Heh, I didn't realize this was a 6 year old post... Oh well, hopefully this helps someone else out there.
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We've had this problem a few times with users using Windows Vista and 7 with Office 2007. It appears to be a problem with how Outlook (or perhaps Windows Vista and 7) handle multiple files with the same filename. Generally when a file gets put into a place where a file with the same name exists Windows will tack a (n) onto the end of it (n being a number, starting at 0). The problem you're experiencing starts when a file exists when n=99. Windows apparently refuses to go any higher than 99, and so you must delete those files.
Outlook saves VoiceMessage.wav in Outlook's temporary downloads folder where it puts all of your attachments. In Vista (and 7, IIRC), that path is C:\Users\\AppData\Local\Temporary Internet Files\Content.Outlook\ (where is a random string of letters and numbers). Generally what I've had to do is close Outlook and go in and delete all of the VoiceMessage.wav files. Once those are gone you can open up Outlook again and everything will be fixed.
I've never had this problem on my own computer, but it does happen to other users on our network. My first thought is that the folder is supposed to be cleared out every once in a while and either that's not happening or they're getting more than 100 VoiceMessages between the times it's supposed to be cleared out.
In my initial troubleshooting I also noticed that Outlook would actually create two VoiceMessage.wav files each time I tried to listen to a VoiceMessage, and once I was done with the file I think it would only delete one copy (I _think_, but can't remember for sure right now). I expect that is the biggest part of the problem.
Hope this helps.
