In programs like XMMS, mplayer, vlc, xine, and many others. You will have to specify surround51 when using the ALSA plugin. Also the sound have to have 5.1 channels in order for 5.1 to work with out tweaking ~/.asoundrc or other ALSA files. If the sound only has 2 channels, you have to edit or create ~/.asoundrc to copy the front channels to the subwoofer, center, and rears. Also you will have to specify the entry that you added to ~/.asoundrc. ALSA has an example in their wiki documentation to setup simulated surround sound playing 2 channel audio files. All channels will be in full-range that a 5.1 channel sound card can handle (about 20 Hz to 20,000 Hz), so you will need to download some plugins that passes low frequencies to the subwoofer channel and add a delay to the rear speakers.
OSS can not output more than 2 channels.
In mplayer make sure you also specify "-channels 6" when using surround51 or else it will not output 6 channels (5.1 channels).