OK, thanks. Before we try anything too exotic, lets take a look at your mixer settings:
!!Amixer output
!!-------------
!!-------Mixer controls for card 0 [I82801DBICH4]
Card hw:0 ‘I82801DBICH4’/‘Intel 82801DB-ICH4 with AD1981B at irq 17’
Mixer name : ‘Analog Devices AD1981B’
Simple mixer control ‘Master’,0
Capabilities: pvolume pswitch
Front Left: Playback 31 [100%] [0.00dB] off]
Front Right: Playback 31 [100%] [0.00dB] off]
Simple mixer control ‘Master Mono’,0
Capabilities: pvolume pvolume-joined pswitch pswitch-joined
Mono: Playback 0 [0%] -46.50dB] [off]
Simple mixer control ‘Video’,0
Capabilities: cswitch cswitch-exclusive
Front Left: Capture [off]
Front Right: Capture [off]
Simple mixer control ‘Aux’,0
Capabilities: pvolume pswitch cswitch cswitch-exclusive
Front Left: Playback 0 [0%] -34.50dB] [off] Capture [off]
Front Right: Playback 0 [0%] -34.50dB] [off] Capture [off]
Simple mixer control ‘External Amplifier’,0
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback on]I highlighted in bold red your Master Volume being muted. I think that should be ON. Also your external amp ON. Its possible that should be OFF. I also coloured read some other mixer settings you could try adjusting < Not sure if they are relevant >