You may need to use something like this in your /.asoundrc to make Alsa output sound to all channels.
pcm.!default { type route slave.pcm surround51 slave.channels 6 ttable.0.0 1 ttable.1.1 1 ttable.0.2 1 ttable.1.3 1 ttable.0.4 0.5 ttable.1.4 0.5 ttable.0.5 0.5 ttable.1.5 0.5 }
One page links to Alsa5.1Notes: