I get all of my voicemails, work and personal, on email. I use Gmail for all of my email. Firefox is really slow (for some reason) when it comes to processing attachments. And even if it were faster, it’d still be slow compared to not having to download an attachment or having the attachment immediately accessible.
So I really want this feature, that appears to only be for Google Talk right now, to work for any WAV file that comes through on an e-mail:
http://mail.google.com/support/bin/answer.py?answer=45756&topic=1565&hl=en