Comment 57 for bug 613809

you have to unmute line playback switch and change line playback volume using alsamixer -c0

no need to change capture source to line since you are using the analog mixer of AC97 codec to mix the input signal from line in to line out

set line playback to 0dB

amixer -c 0 set Line playback 72% unmute

the output volume is controlled by master playback volume and switch

Simple mixer control 'Line',0
  Capabilities: pvolume pswitch pswitch-joined cswitch cswitch-exclusive penum
  Capture exclusive group: 0
  Playback channels: Front Left - Front Right
  Capture channels: Front Left - Front Right
  Limits: Playback 0 - 31
  Front Left: Playback 0 [0%] [-34.50dB] [off] Capture [off]
  Front Right: Playback 0 [0%] [-34.50dB] [off] Capture [off]

control.18 {
  iface MIXER
  name 'Line Playback Switch'
  value false
  comment {
   access 'read write'
   type BOOLEAN
   count 1
  }
 }
 control.19 {
  iface MIXER
  name 'Line Playback Volume'
  value.0 0
  value.1 0
  comment {
   access 'read write'
   type INTEGER
   count 2
   range '0 - 31'
   dbmin -3450
   dbmax 1200
   dbvalue.0 -3450
   dbvalue.1 -3450
  }
 }