Getting the same effect on 12.04 , started happening after some update in 2013 january 

When i plug in headphones or pull them out then the machine goes on mute and i have to restore the sound from alsamixer

$ dpkg -l | grep alsa
ii  alsa-base                              1.0.25+dfsg-0ubuntu1                                ALSA driver configuration files
ii  alsa-utils                             1.0.25-1ubuntu5                                     Utilities for configuring and using ALSA

$ dpkg -l | grep pulse
ii  gstreamer0.10-pulseaudio               0.10.31-1ubuntu1                                    GStreamer plugin for PulseAudio
ii  libcanberra-pulse                      0.28-3ubuntu3                                       PulseAudio backend for libcanberra
ii  libpulse-mainloop-glib0                1:1.1-0ubuntu15.2                                   PulseAudio client libraries (glib support)
ii  libpulse-mainloop-glib0:i386           1:1.1-0ubuntu15.2                                   PulseAudio client libraries (glib support)
ii  libpulse0                              1:1.1-0ubuntu15.2                                   PulseAudio client libraries
ii  libpulse0:i386                         1:1.1-0ubuntu15.2                                   PulseAudio client libraries
ii  libpulsedsp                            1:1.1-0ubuntu15.2                                   PulseAudio OSS pre-load library
ii  libpulsedsp:i386                       1:1.1-0ubuntu15.2                                   PulseAudio OSS pre-load library
rc  libsdl1.2debian-pulseaudio             1.2.14-6.1ubuntu4                                   Simple DirectMedia Layer (with X11 and PulseAudio options)
ii  pulseaudio                             1:1.1-0ubuntu15.2                                   PulseAudio sound server
ii  pulseaudio-esound-compat               1:1.1-0ubuntu15.2                                   PulseAudio ESD compatibility layer
ii  pulseaudio-module-bluetooth            1:1.1-0ubuntu15.2                                   Bluetooth module for PulseAudio sound server
ii  pulseaudio-module-gconf                1:1.1-0ubuntu15.2                                   GConf module for PulseAudio sound server
ii  pulseaudio-module-x11                  1:1.1-0ubuntu15.2                                   X11 module for PulseAudio sound server
ii  pulseaudio-utils                       1:1.1-0ubuntu15.2                                   Command line tools for the PulseAudio sound server
ii  vlc-plugin-pulse                       2.0.5-0ubuntu0.12.04.1                              PulseAudio plugin for VLC

$ cat /proc/asound/cards
 0 [Generic        ]: HDA-Intel - HD-Audio Generic
                      HD-Audio Generic at 0xf0444000 irq 53
 1 [Generic_1      ]: HDA-Intel - HD-Audio Generic
                      HD-Audio Generic at 0xf0440000 irq 16

it seems that card 0 is my ati hdmi and card 1 is the actual soundcard on the motherboard 

Laptop : HP Pavilion dv6


$ lspci -v | grep Audio -A2
00:01.1 Audio device: Advanced Micro Devices [AMD] nee ATI BeaverCreek HDMI Audio [Radeon HD 6500D and 6400G-6600G series]
	Subsystem: Hewlett-Packard Company Device 358d
	Flags: bus master, fast devsel, latency 0, IRQ 53
--
00:14.2 Audio device: Advanced Micro Devices [AMD] Hudson Azalia Controller (rev 01)
	Subsystem: Hewlett-Packard Company Device 358d
	Flags: bus master, slow devsel, latency 32, IRQ 16


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