- speakers do not output _any_ sound whatsoever anymore
Are you using PulseAudio or without? Try to check without PA at first.
I never use PA, so this is all with plain ALSA.
In anyway, get alsa-info.sh outputs at the headphone plugged and unplugged on both 3.17.x and 3.18.x kernels. Run the script with --no-upload option and attach them.
If Combo audio connector does not support conventional microphone, this mean the driver should not create mic jack control for node 0x1a
http://support.lenovo.com/us/en/documents/pd029567
Connect headphones or a headset that has a 3.5-mm (0.14-inch), 4-pole plug to the combo audio connector to listen to the sound from the computer.
Notes : • If you are using a headset with a function switch, for example, a headset for an iPhone mobile digital device, do not press this switch while using the headset. If you press the switch, the headset microphone will be disabled, and the integrated microphones on the computer will be enabled instead.
• The combo audio connector does not support a conventional microphone.