You have to make sure ALSA is used instead of oss. Next make sure the output device is either spdif or iec958. This is far I can go because I do not have the equipment to convert spdif or iec958 to analog. My sound card's, Audiotrak Prodigy 7.1, spdif or iec958 output is hw:0,1 because hw:0,0 and hw:0,2 are front, rear, center, sides, and subwoofer.
A good howto is at http://gentoo-wiki.com/HOWTO_Dolby_D...t_(AC3,_SPDIF)
The USE flags are compile options that can be passed to the configure utility. If you installed mplayer or kaffeine (xine-lib) through an rpm, deb, or other packaged based form, AC3 or DTS may not be included.