Mplayer has enough configuration arguments to initiate surround sound from stereo audio. One of my earlier posts was on how to activate the subwoofer channel. Now here is the full configuration
$ gmplayer -ao alsa -af surround=10,sub=75:5,center=4 -channels 5
surround=10 : specifies the delay in surround channels (here it is 10)
center=4 : Which channel to redirect center channel audio derived from the front channels.
Mplayer will then derive the rest of the channels from the front channels and we have 5.1 music.