How did you run alsamixer ?
By default alsamixer will run on the virtual “PulseAudio” device, to be able to actual change the mixer-settings of your USB microphone you need to select the right soundcard, either running alsamixer -c # where # is 1, 2, 3, … or by pressing F6 and selecting the right soundcard. Also you can try pressing F5 to see all available controls for your USB microphone card.
If you were already controlling the right sound-card and things are still too soft then I’m afraid you are out of luck. Alsamixer controls the cards controls at the lowest level, so if that does not work, then there is not much else that you can do AFAIK.