Using an intel integrated audio chipset:
00:1b.0 Audio device: Intel Corporation N10/ICH 7 Family High Definition Audio Controller (rev 01)
..and struggling for about a year to unmute it (to use with skype) using amixer as opposed to alsamixer, I finally hit on:
$ amixer set Capture 80% toggle
..really confusingly, the device is not called "Mic" and "mute"/"unmute" don't seem to do anything.
'Hope this is useful for somebody else