hi piper, here is the output
absolut@sidbox: ~ $ amixer -c 0 | grep mixer
Simple mixer control 'IEC958',0
Simple mixer control 'IEC958',1
Simple mixer control 'IEC958',2
Simple mixer control 'IEC958',3
Simple mixer control 'IEC958',4
absolut@sidbox: ~ $ amixer -c 1 | grep mixer
Simple mixer control 'Master',0
Simple mixer control 'Headphone',0
Simple mixer control 'PCM',0
Simple mixer control 'Front',0
Simple mixer control 'Front Mic',0
Simple mixer control 'Front Mic Boost',0
Simple mixer control 'Surround',0
Simple mixer control 'Center',0
Simple mixer control 'LFE',0
Simple mixer control 'Line',0
Simple mixer control 'Line Boost',0
Simple mixer control 'IEC958',0
Simple mixer control 'IEC958 Default PCM',0
Simple mixer control 'Capture',0
Simple mixer control 'Capture',1
Simple mixer control 'Digital',0
Simple mixer control 'Input Source',0
Simple mixer control 'Input Source',1
Simple mixer control 'Loopback Mixing',0
Simple mixer control 'Rear Mic',0
Simple mixer control 'Rear Mic Boost',0
in case this is relevant, the hardware setup looks as follows:
computer (onboard sound) ---> optical cable ---> AV receiver ---> loudspeakers