Pretty sure there is no application anywhere at this point that makes VoIP calls come through the phone speaker. I read somewhere that it's a fundamental issue with access to the speaker in windows mobile. This was the case on my apache with Microsoft VoIP client, on my Vogue with Fring... and I haven't bothered on my diamond yet (because, like rainfreak, I also have more phone minutes than I need, so it's purely academic).
The solution is to use a headset. It's hacky, but it gets the job done.
And, as far as getting the built in Microsoft VoIP working, I think this xda post should point you in the right direction. (I haven't actually played with it and don't know if there're any caveats for CDMA diamonds; but, if there are, there are answers down that xda rabbithole, undoubtedly).
http://forum.xda-developers.com/showthread.php?t=415262