So I have searched about A2DP but this seems to be the opposite...

I would like to have the n800 hooked up to a stereo across the room/in another room and would like to send music from my computer to it.

any way to do this?

basically, I want to make the n800 work like apple's airport express, so any other way or app that can do this would be great.

As far as I know no one has come up with a way to use the tablets as bluetooth speakers. Assuming you have a wireless connection, you could use VLC or mplayer to stream the audio to your tablet. On my Windows computer I can select the "stereo pin" as an audio capture device in VLC, so that any audio on the computer is streamed. Then you can use mplayer to play the stream on your tablet.