Comment 4 for bug 669958

It isn't possible to unmute the mic(s) by default, because there may be horrendous feedback. You should check the capture level in Sound Preferences' Input tab.

See also the following mixer control settings:

Simple mixer control 'Mic',0
  Capabilities: pvolume pvolume-joined pswitch pswitch-joined cswitch cswitch-exclusive penum
  Capture exclusive group: 0
  Playback channels: Mono
  Capture channels: Front Left - Front Right
  Limits: Playback 0 - 31
  Mono: Playback 16 [52%] [-10.50dB] [on]
  Front Left: Capture [off] <---
  Front Right: Capture [off] <---
Simple mixer control 'Mic Boost (+20dB)',0
  Capabilities: pswitch pswitch-joined penum
  Playback channels: Mono
  Mono: Playback [off] <---
Simple mixer control 'Mic Front Input',0
  Capabilities: pswitch pswitch-joined penum
  Playback channels: Mono
  Mono: Playback [off]
Simple mixer control 'Mic Select',0
  Capabilities: enum
  Items: 'Mic1' 'Mic2'
  Item0: 'Mic1'
[...]
Simple mixer control 'Capture',0
  Capabilities: cvolume cswitch cswitch-joined penum
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 15
  Front Left: Capture 15 [100%] [22.50dB] [off] <---
  Front Right: Capture 15 [100%] [22.50dB] [off] <---