I'm not the expert here, but if I were you, I would run
amixer -D hw:x contents amixer -D hw:x scontents
To see if your controls show up there. If it shows up in scontents, that would be the "simple" mixer interface that PulseAudio uses. If not, using the control api (as being done by "contents") should work.
Anyway, reading the source code to amixer would then be the first step towards understanding how to enumerate and read mixer controls.
That part was done already. I was looking at PulseAudio integration. By looking at additional source code, I found out I could retrieve the card and device number with snd_pcm_info_get_card() and snd_pcm_info_get_device(). I can now read the ELD control from PulseAudio, all I need is to add the ELD parsing and set the codecs supported by the receiver accordingly. -Pierre