Firstly, this article requires version 6.00 or higher of the advanced or standard codec pack, or a recent and stable version of ffdshow.

Click "Start" and go to "Programs", then "Cole2k Media - Codec Pack" in your start menu, select "ffdshow Audio" and you should see a window open like that shown in the first picture.

In the codecs pane you will see a list of formats, what you need to do now is set the options in the decoder column as follows:

AC3 to "S/PDIF"
DTS to "S/PDIF"
AAC to "realaac"
Vorbis to "libavcodec"

Finally, just below the "Mixer" option in the left hand pane select the "Output" option, as shown in the third picture.
Set "Supported output sample formats" to "AC3" and set the bitrate next to it to "384".

You should now be able to enjoy AC3, DTS, AAC, OGG mutli channel files in full.

Notes:
During playback AC3, AAC and OGG will all appear as AC3 on an external decoder, DTS will appear as DTS.
This method does pass through Dolby Digital EX and DTS-ES.
In some cases ffdshow will fail to decode 2 channel AAC files.