frumble did you enable the 5.1 profile in pulseaudio? I'm not sure if this is possible (don't have rpi handy)
but something like this
will show something like
Card #0
Name: alsa_card.pci-0000_00_08.0
Driver: module-alsa-card.c
Owner Module: 6
Properties:
alsa.card = "0"
alsa.card_name = "HDA NVidia"
<snip>
.......
</snip>
device.icon_name = "audio-card-pci"
Profiles:
input:analog-stereo: Analog Stereo Input (sinks: 0, sources: 1, priority: 60, available: yes)
output:analog-stereo: Analog Stereo Output (sinks: 1, sources: 0, priority: 6000, available: yes)
output:analog-stereo+input:analog-stereo: Analog Stereo Duplex (sinks: 1, sources: 1, priority: 6060, available: yes)
output:analog-surround-21: Analog Surround 2.1 Output (sinks: 1, sources: 0, priority: 800, available: yes)
output:analog-surround-21+input:analog-stereo: Analog Surround 2.1 Output + Analog Stereo Input (sinks: 1, sources: 1, priority: 860, available: yes)
output:analog-surround-40: Analog Surround 4.0 Output (sinks: 1, sources: 0, priority: 700, available: yes)
output:analog-surround-40+input:analog-stereo: Analog Surround 4.0 Output + Analog Stereo Input (sinks: 1, sources: 1, priority: 760, available: yes)
Display More
then do something like
pactl set-card-profile 0 output:analog-surround-40
the "0" being the card number as given by "pactl list cards."