Comment # 2 on bug 1172412 from Takashi Iwai

The percentage is a bogus number and cannot be used for judging the loudness in
general.  The USB audio specifies the volume in 1/100 dB unit, and most likely
your firmware is buggy and provides a wrong dB value for the playback volume.

Looking at the alsa-info.sh output:
state.Quadcast {
.....
    control.6 {
        iface MIXER
        name 'Speaker Playback Volume'
        value.0 1
        value.1 1
        comment {
            access 'read write'
            type INTEGER
            count 2
            range '0 - 31'
            dbmin -4000
            dbmax -900
            dbvalue.0 -3900
            dbvalue.1 -3900
        }
    }

So the control receives the raw value from 0 to 31 corresponding the dB level
from -40dB to -9dB.  This looks already strange.

Could you check whether you get any signal if you set the raw value greater
than 0?  e.g.

  amixer -c0 cset name='Speaker Playback Volume' 0,0

should set the values to 0 and it's likely the silence.  Then

  amixer -c0 cset name='Speaker Playback Volume' 1,1

and check whether you get some output at all.  Then increase the value and
confirm that you have higher corresponding volume.

Then the next step is identify the real loudness (in dB level) to each raw
value.  At best, compare the output level with other device.  You might be able
to compare with the output from the onboard audio, too.