Same issue, same machine as Peter.  Dell 5510, upgraded to 16.04 --  at 1st the speakers worked, but the headphone would not.   Now the headphones work, but the internal speakers don't.


==================== HEADPHONE PLUGGED IN ====================

$ pactl list sinks
Sink #0
	State: SUSPENDED
	Name: alsa_output.pci-0000_00_1f.3.analog-stereo
	Description: Built-in Audio Analog Stereo
	Driver: module-alsa-card.c
	Sample Specification: s16le 2ch 48000Hz
	Channel Map: front-left,front-right
	Owner Module: 6
	Mute: no
	Volume: front-left: 65536 / 100% / 0.00 dB,   front-right: 65536 / 100% / 0.00 dB
	        balance 0.00
	Base Volume: 65536 / 100% / 0.00 dB
	Monitor Source: alsa_output.pci-0000_00_1f.3.analog-stereo.monitor
	Latency: 0 usec, configured 0 usec
	Flags: HARDWARE DECIBEL_VOLUME LATENCY 
	Properties:
		alsa.resolution_bits = "16"
		device.api = "alsa"
		device.class = "sound"
		alsa.class = "generic"
		alsa.subclass = "generic-mix"
		alsa.name = "ALC3266 Analog"
		alsa.id = "ALC3266 Analog"
		alsa.subdevice = "0"
		alsa.subdevice_name = "subdevice #0"
		alsa.device = "0"
		alsa.card = "0"
		alsa.card_name = "HDA Intel PCH"
		alsa.long_card_name = "HDA Intel PCH at 0xddd28000 irq 137"
		alsa.driver_name = "snd_hda_intel"
		device.bus_path = "pci-0000:00:1f.3"
		sysfs.path = "/devices/pci0000:00/0000:00:1f.3/sound/card0"
		device.bus = "pci"
		device.vendor.id = "8086"
		device.vendor.name = "Intel Corporation"
		device.product.id = "a170"
		device.product.name = "Sunrise Point-H HD Audio"
		device.form_factor = "internal"
		device.string = "front:0"
		device.buffering.buffer_size = "65536"
		device.buffering.fragment_size = "32768"
		device.access_mode = "mmap+timer"
		device.profile.name = "analog-stereo"
		device.profile.description = "Analog Stereo"
		device.description = "Built-in Audio Analog Stereo"
		alsa.mixer_name = "Realtek ALC3266"
		alsa.components = "HDA:10ec0298,102806e5,00100103 HDA:80862809,80860101,00100000"
		module-udev-detect.discovered = "1"
		device.icon_name = "audio-card-pci"
	Formats:
		pcm



======================   HEADPHONES UNPLUGGED =====================
$ pactl list sinks
Sink #0
	State: SUSPENDED
	Name: alsa_output.pci-0000_00_1f.3.analog-stereo
	Description: Built-in Audio Analog Stereo
	Driver: module-alsa-card.c
	Sample Specification: s16le 2ch 48000Hz
	Channel Map: front-left,front-right
	Owner Module: 6
	Mute: no
	Volume: front-left: 65536 / 100% / 0.00 dB,   front-right: 65536 / 100% / 0.00 dB
	        balance 0.00
	Base Volume: 65536 / 100% / 0.00 dB
	Monitor Source: alsa_output.pci-0000_00_1f.3.analog-stereo.monitor
	Latency: 0 usec, configured 0 usec
	Flags: HARDWARE DECIBEL_VOLUME LATENCY 
	Properties:
		alsa.resolution_bits = "16"
		device.api = "alsa"
		device.class = "sound"
		alsa.class = "generic"
		alsa.subclass = "generic-mix"
		alsa.name = "ALC3266 Analog"
		alsa.id = "ALC3266 Analog"
		alsa.subdevice = "0"
		alsa.subdevice_name = "subdevice #0"
		alsa.device = "0"
		alsa.card = "0"
		alsa.card_name = "HDA Intel PCH"
		alsa.long_card_name = "HDA Intel PCH at 0xddd28000 irq 137"
		alsa.driver_name = "snd_hda_intel"
		device.bus_path = "pci-0000:00:1f.3"
		sysfs.path = "/devices/pci0000:00/0000:00:1f.3/sound/card0"
		device.bus = "pci"
		device.vendor.id = "8086"
		device.vendor.name = "Intel Corporation"
		device.product.id = "a170"
		device.product.name = "Sunrise Point-H HD Audio"
		device.form_factor = "internal"
		device.string = "front:0"
		device.buffering.buffer_size = "65536"
		device.buffering.fragment_size = "32768"
		device.access_mode = "mmap+timer"
		device.profile.name = "analog-stereo"
		device.profile.description = "Analog Stereo"
		device.description = "Built-in Audio Analog Stereo"
		alsa.mixer_name = "Realtek ALC3266"
		alsa.components = "HDA:10ec0298,102806e5,00100103 HDA:80862809,80860101,00100000"
		module-udev-detect.discovered = "1"
		device.icon_name = "audio-card-pci"
	Formats:
		pcm