@David Henningsson:

1. I had installed those, and indeed today they were updated by Update Manager (which prompted me to check back here).

2. I found the same things as Gargoulf, so rebooted into Recovery Mode and dropped into a root command prompt, where the commands worked. (The machine paused for a second after the reconfig command, so I assume it was doing it -- are these the Linux equivalent of hardware pokes?)

Rebooting shows that nothing has changed:

System / Preferences / Sound / Input / Connector still shows the same four options: 
  Microphone 1 / Microphone  
  Microphone 1 / Line-In
  Microphone 2 / Microphone  
  Microphone 2 / Line-In
and selecting any of them does not respond to the inbuilt mic.

From the terminal, 'alsamixer' gives a whole lot of information (none of which helps):

Under F4 Capture it has Front Mic, Mic, Capture and Capture 1. Putting the gain to full on all of them doesn't help. Options for Input Source and Input Source 1 are Front Mic and Mic. Switching between them on either doesn't help.

Under F2 System information I get these (apologies for the format -- I'm cutting and pasting from the terminal-mode graphics):


│         │▒▒│     ┌────────────────── /proc/asound/version ───────────────────┐                   │
│         │▒▒│     │Advanced Linux Sound Architecture Driver Version 1.0.23.   │                   │
│         │▒▒│     │Compiled on Aug 13 2010 for kernel 2.6.32-24-generic (SMP).│                   │
│         │▒▒│     └───────────────────────────────────────────────────────────┘                   │


│         │▒▒│         ┌──────────────── /proc/asound/cards ────────────────┐                      │
│         │▒▒│         │ 0 [Intel          ]: HDA-Intel - HDA Intel         │                      │
│         │▒▒│         │                      HDA Intel at 0xf6ffc000 irq 40│                      │
│         │▒▒│         └────────────────────────────────────────────────────┘                      │

│         │▒▒│           │▒▒│  ┌─────── /proc/asound/devices ───────┐                              │
│         │▒▒│           │▒▒│  │  2:        : timer                 │                              │
│         │▒▒│           │▒▒│  │  3:        : sequencer             │                              │
│         │▒▒│           │▒▒│  │  4: [ 0- 1]: digital audio playback│                              │
│         │▒▒│           │▒▒│  │  5: [ 0- 0]: digital audio playback│                              │
│         │▒▒│           │▒▒│  │  6: [ 0- 0]: digital audio capture │                              │
│         │▒▒│           │▒▒│  │  7: [ 0- 0]: hardware dependent    │                              │
│         │▒▒│           │▒▒│  │  8: [ 0]   : control               │                              │
│         │▒▒│           │▒▒│  └────────────────────────────────────┘                              │
││         │▒▒│           │▒▒│       ┌ /proc/asound/oss/devices ┐                                   │
│         │▒▒│           │▒▒│       │  0: [0- 0]: mixer        │                                   │
│         │▒▒│           │▒▒│       │  1:       : sequencer    │                                   │
│         │▒▒│           │▒▒│       │  3: [0- 0]: digital audio│                                   │
│         │▒▒│           │▒▒│       │  4: [0- 0]: digital audio│                                   │
│         │▒▒│           │▒▒│       │  8:       : sequencer    │                                   │
│         │▒▒│           │▒▒│       │ 12: [0- 1]: digital audio│                                   │
│         │▒▒│           │▒▒│       └──────────────────────────┘                                   │


│         │▒▒│           ┌───────────── /proc/asound/timers ─────────────┐                         │
│         │▒▒│           │G0: system timer : 10000.000us (10000000 ticks)│                         │
│         │▒▒│           │P0-0-0: PCM playback 0-0-0 : SLAVE             │                         │
│         │▒▒│           │P0-0-1: PCM capture 0-0-1 : SLAVE              │                         │
│         │▒▒│           │P0-0-3: PCM capture 0-0-3 : SLAVE              │                         │
│         │▒▒│           │P0-1-0: PCM playback 0-1-0 : SLAVE             │                         │
│         │▒▒│           └───────────────────────────────────────────────┘                         │

│         │▒▒│  ┌─────────────────────── /proc/asound/pcm ────────────────────────┐                │
│         │▒▒│  │00-00: STAC92xx Analog : STAC92xx Analog : playback 1 : capture 2│                │
│         │▒▒│  │00-01: STAC92xx Digital : STAC92xx Digital : playback 1          │                │
│         │▒▒│  └─────────────────────────────────────────────────────────────────┘                │


Any help greatly appreciated. 

John