Why not use winamp? From what I remember it was pretty universal and supported almost every format.
As far as media monkey, it has a server add-on, which would allow for controlling it from your playbook through the browser. Or, for more fun, there is the media streamer server which would allow your media monkey to stream to your playbook via the browser.
Looking through the site, it says it uses the same standard as itunes, which COULD mean that you could pick up the RemoteTunes app, and use that. However, I dont have itunes, or media monkey, so I dont see a reason for me to try it. =p