If you go into the Asterisk voicemail configuration file (/etc/asterisk/voicemail.conf on a regular Asterisk or /opt/asterisk/etc/voicemail.conf if you're running Asterisk on the N900 itself) you will find a line in the [general] section that looks like this:

format=wav49|gsm|wav

Change it to:

format=wav|wav49|gsm

In order to get Asterisk to work with this new configuration, get on to your Asterisk command line and issue the following command:

*CLI> module reload app_voicemail.so

...and you should now find that your voicemail attachments will play on the N900 media player.

One more thing: The first time you do this you may experience continual repetition of the message, as (for whatever reason) the media play comes up in auto-repeat mode (coloured blue when on) - pictured below.

All you need to do is touch the auto-repeat symbol to turn it off (the symbol will go white) and this setting will be remembered for the future.

So now your Asterisk voicemails should playback perfectly!

BTW - these files should also play on desktops (Windows tested successfully) so this appears to be an all-round solution.