Well this thing is weird. I have no problems playing divx/xvids which are using ac3 encoded sound. Sound is coming through SPDIF with no problems to my receiver Yamaha RX-V630RDS and played as it is (DTS/Dolby Digital etc.) But when I'm trying to play DVD with Dolby Digital 5.1 or DTS there won't be no sound. Player I'm using is Media Player Classic and audio decoder ffdshow. AC3 is set on from settings. From internal Filters all but MPEG-2 Video are disabled. Soundcard of mine is Audigy 2 with lastest drivers. Also from soundcard Device Controls, Decoder is set to SPDIF. DVD drive is LG GCC-4521B. Windows XP Pro SP2. Creepy..
Download and run AC3Filter which is made specifically to play ac3 from movies http://www.afterdawn.com/software/video_software/codecs_and_filters/ac3filter.cfm Enjoy!