I think media car uses Winamp so you'll need to get Winamp to play in 5.1
I´ve just bought everything I need for my carpc. Athlon XP2000, 80Gb, 802.11g, 512DDr, lilli, and 5.1 Genius PCI audio card.
I install all drivers and try with WMP and 5.1 works very nice. But when I try MediaCar I have audio only in the fronts speakers. Please help