Maybe something like
http://www.pocketpccity.com/software...-pocketpc.html
Not free, but audio message exchange over bluetooth sounds like a nice low power solution with reasonable range. Just so long as you use X50's with WM2003se and not WM5 (bluetooth suckiness issue).....
-=Edit=-
Possibly better to go to the source.
http://www.buzzone.net/eng/