pi@netaudio:~ $ sudo amixer -c0 controls
numid=3,iface=PCM,name='Capture Channel Map'
numid=1,iface=PCM,name='Playback Channel Map'
numid=2,iface=PCM,name='Playback Channel Map',device=1

pi@netaudio:~ $ sudo amixer -c0 contents
numid=3,iface=PCM,name='Capture Channel Map'
  ; type=INTEGER,access=r----R--,values=2,min=0,max=36,step=0
  : values=0,0
  | container
    | chmap-fixed=FL,FR

numid=1,iface=PCM,name='Playback Channel Map'
  ; type=INTEGER,access=r----R--,values=2,min=0,max=36,step=0
  : values=3,4
  | container
    | chmap-fixed=FL,FR

numid=2,iface=PCM,name='Playback Channel Map',device=1
  ; type=INTEGER,access=r----R--,values=1,min=0,max=36,step=0
  : values=0
  | container
    | chmap-fixed=MONO