Unexpectedly lost speaker output. Headphone output works fine

Question

Yesterday when I opened my laptop I noticed that I no longer had sound from the built-in speakers. I carefully checked settings in Sound Preferences, and then dropped to shell and looked in alsamixer. Nothing is showing muted or 0/no volume. Everything is on and set to max volume. The profile is Analog Stereo Duplex, the default.

Sound works fine when using headphones.

I compared my alsa conf files with samples from known good systems and am not finding any difference.

I'm runnning out of ideas and even updated my kernel to 3.8.8 in hopes that would shake this loose. No dice there, so I purged that and am back to the factory kernel 3.5.0.

Here are my current specs:

RELEASE=14
CODENAME=nadia
EDITION="MATE 64-bit"
DESCRIPTION="Linux Mint 14 Nadia"
DESKTOP=MATE
TOOLKIT=GTK

lspci:

00:1b.0 Audio device: Intel Corporation 7 Series/C210 Series Chipset Family High Definition Audio Controller (rev 04)

cat /proc/asound/card0/codec#* | grep Codec

Codec: IDT 92HD91BXX
Codec: Intel PantherPoint HDMI
astembridge-Inspiron-5720

aplay -l

**** List of PLAYBACK Hardware Devices ****
card 0: PCH [HDA Intel PCH], device 0: STAC92xx Analog [STAC92xx Analog]
  Subdevices: 1/1
  Subdevice #0: subdevice #0
card 0: PCH [HDA Intel PCH], device 3: HDMI 0 [HDMI 0]
  Subdevices: 0/1
  Subdevice #0: subdevice #0

Alsamixer screengrab:

If I need to post more here please indicate in a comment.

EDIT 1: The laptop itself is less than a month old and never dropped or otherwise mistreated. It is a Dell Inspiron 17R - 5720.

EDIT 2: (March 2016 update). Got this old laptop out again. I installed a fresh copy of Win7 yesterday in hopes it might have been a weird nix issue. Nope. Windows showed the audio driver installed and working properly, and when I played a track I can see the sound meter bouncing up & down. Just no audio from either speakers or headphone jack.

Answer 1

There is a "power" amplifier that drives the speakers which is a distinct hardware circuit from that which drives the headphones. This circuit is also analog which would mean that the aplay output would find the audio chip operating just fine and the power amp is not instrumented for software detection.

Given then timing of "under a month", this falls well into the realm of "infant hardware failure" and should be under warranty. Given that this is the only detectable failure right now, does not mean that it isn't symptomatic of a larger defect in "workmanship or material" which will brick your machine after the warranty period.

Yeah, it's a pain to get warranty service, but probably less of a pain of having a dead machine after a little more burn-in.

You say "No broken, frayed or damaged wires" which doesn't include bad circuit board traces, blown capacitors, improperly soldered op-amps, etc., etc. all of which would be invisible even if you did open the box.