Use FFDSHOWs audio config to re-encode in real time to 5.1 for VIDEO, and use Alex AC3 filter for re-encoding to 5.1 for mp3. (mp3 5.1 works on analog output only)
Video output however also works on spdif out (unstable if u use 2 displays)
If you are going to use analog out, the setup works okay.
Get the K-lite codec pack and import these settings, see attachment
On Alex Ac3 filter, leave it default except from set it to 5.1 instead of 2.0