In a perfect world we would use an open licensed compressed codec that came pre-installed on the majority of platforms.

This is the essence of my first query. While I want to be able to hear the voicemail on my n900, I also want to be able to hear it on any old machine that I happen across and can get to my email with. And the vast majority of those will be Windows, and will not have uncontained gsm or speex codec installed. They will however be able to decode wav49, as do most linux distros out of the box.

So the fastest if not best route to getting a cross platform compressed voicemail format for asterisk that will work with the n900 is to get the n900 to support wav49