I found this out about AC3 filters:
When you playback an AVI-AC3, you may want to choose whether the AC-3 sound should be decoded to 2-channels, 4-channels, 6-channels, or passed-through via your S/PDIF output (to be processed by a AC-3 receiver). However, to be able to make such a choice, your video player must give you the option of changing the properties of the DirectShow filters it is using. Windows Media Player 6 offers that option, while the version 7 doesn't. I don't know about other video players. If you can't change the options, you will still get sound, but most likely it will be converted from 5.1 to 2-channels.
Yep, that's my problem, I can't configure it to give 6 channels. Am I SOL with Mediacar?