I have a patch for this problem kern/15441, 
and I have already appended this to the NetBSD GNATS database.
But I forgot to carbon-copy.

http://www.NetBSD.org/cgi-bin/query-pr-single.pl?number=15441

This patch recognizes input and output terminal types
connected to units (feature  etc.),
analyzes them, and names feature units.
In the case of `Sound Blaster Digital Music', for example,
`mixerctl -a` outputs

outputs.master.mute=off
outputs.speaker.mute=off
outputs.master=0,0
outputs.speaker=0,0
record.mic.mute=off
record.mic=2
record.mic.agc=off
record.line.mute=off
record.line=2,2
record.aux.mute=off
inputs.mic.mute=off
inputs.mic=192
inputs.line.mute=on
inputs.line=192,192

I have `Sound Blaster Digital Music' only.
So I had to make this system for unit included in this
(input and output terminal, feature, selector, and mixer unit),
and for feature controls (mute, volume, agc) only.
But I believe that it is easy 
to improve this for other units and controls.