Comment 39 for bug 1040873

not sure why the subwoofer use TRRS Jack

does it use spdif since 3 pins

http://www.intel.com/support/motherboards/desktop/sb/CS-032871.htm

there is also a mono pin

Node 0x17 [Pin Complex] wcaps 0x40050c: Mono Amp-Out
  Amp-Out caps: ofs=0x00, nsteps=0x00, stepsize=0x00, mute=1
  Amp-Out vals: [0x80]
  Pincap 0x00000010: OUT
  Pin Default 0x411111f0: [N/A] Speaker at Ext Rear
    Conn = 1/8, Color = Black
    DefAssociation = 0xf, Sequence = 0x0
    Misc = NO_PRESENCE
  Pin-ctls: 0x00:
  Power states: D0 D1 D2 D3 EPSS
  Power: setting=D0, actual=D0
  Connection: 1
     0x0f

if subwoofer is only connected to right channel of node 0x16, the solution is force the driver in 4 channel mode and need pulseaudio to mix left and right to subwoofer

you can also restrict the auto parser to put the two speaker pins into extra out

but those notebook with three internal speakers to line out to support 5.1

for (i = 0; i < ARRAY_SIZE(mout->extra_out_nid); i++)
  if (!mout->no_share_stream && mout->extra_out_nid[i])
   snd_hda_codec_setup_stream(codec,
         mout->extra_out_nid[i],
         stream_tag, 0, format);

the retask of spdif to speaker won`t help since you lose the center and rear channels in 5.1 mode

 it is just use a bug in the auto parser and those alc880 codec with zero pin default at the digital pin complex