Comment 18 for bug 1251127

With alsa there is a ""Simple mixer control 'Independent HP'" if set to
  * "Enable", sound is /only/ audible through the built-in speaker
  * "Disable", sound is /only/ audible through the connected headphone/line-out

as pulseaudio only use device 0 for analog playback,

when independent HP is on , HP is only used for alt analog device 2 (auto mute of speaker by headphone is disabled)

when independent HP is off, HP is used by analog device 0

if your external speaker is always plugged into line out,

do you mean you want the internal speaker only auto muted by headphone but not by line out ?

APLAY

**** List of PLAYBACK Hardware Devices ****
card 0: Intel [HDA Intel], device 0: AD1884 Analog [AD1884 Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: Intel [HDA Intel], device 2: AD1884 Alt Analog [AD1884 Alt Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0

Node 0x03 [Audio Output] wcaps 0x405: Stereo Amp-Out
  Control: name="Headphone Playback Volume", index=0, device=0
    ControlAmp: chs=3, dir=Out, idx=0, ofs=0
  Device: name="AD1884 Alt Analog", type="Audio", device=2
  Amp-Out caps: ofs=0x27, nsteps=0x27, stepsize=0x05, mute=0
  Amp-Out vals: [0x27 0x27]
  Converter: stream=0, channel=0
  Power states: D0 D3
  Power: setting=D0, actual=D0
Node 0x04 [Audio Output] wcaps 0x405: Stereo Amp-Out
  Control: name="PCM Playback Volume", index=0, device=0
    ControlAmp: chs=3, dir=Out, idx=0, ofs=0
  Device: name="AD1884 Analog", type="Audio", device=0
  Amp-Out caps: ofs=0x27, nsteps=0x27, stepsize=0x05, mute=0
  Amp-Out vals: [0x27 0x27]
  Converter: stream=0, channel=0
  Power states: D0 D3
  Power: setting=D0, actual=D0