I had a problem with my iMic because the HW mixer was associated to the Bass control, so I logged in via SSH and with “alsamixer” I checked which controls were available. I saw “PCM” so I changed mpd.conf with
mixer_control “PCM”
and now it works.
I suggest placing such control in the Playback preferences, since not everyone would be able to do the same and you cannot guarantee that the first mixer device picked up by ALSA will be the volume.