The command-line tool alsamixer can be used to control the volume of the USB audio output:
Unfortunately the setting does not survive a reboot. Another tool from the alsa-utils package can be used for this purpose: alsactl
After setting the volume with alsamixer, the following command persists the current setting in the file-system for device 0:
alsactl -f /etc/alsa/alsa0.state store 0
During startup, the state must be restored again. This can be achieved by adding the following command to /etc/rc.local:
alsactl -f /etc/alsa/alsa0.state restore 0