Comment 37 for bug 871808

So, if you want automatic LFE remixing, that should be possible through PulseAudio:

First, add a new profile for 2.1 by adding this section to /usr/share/pulseaudio/alsa-mixer/profile-sets/default.conf:

[Mapping analog-surround-21]
# Note that the line below should NOT be changed to surround21.
device-strings = surround40:%f
channel-map = front-left,front-right,lfe,lfe
paths-output = analog-output analog-output-speaker
priority = 7
direction = output

Second, make sure this line is present in /etc/pulse/daemon.conf:

enable-lfe-remixing = yes

Then restart pulseaudio (run "pulseaudio -k" in a terminal), then select the new profile in the sound settings.

I haven't used LFE remixing myself, so not sure how well it works, but worth a try :-)