The VM9312 has a volume knob on the unit's faceplate. Just rotate the knob on the left (a little below the optical disc D slot) clockwise to increase the volume. Turn it the opposite way to lower the volume. (If you have the knob set to the maximum volume and then lowered the volume with the remote, you may need to reset the unit; see below.) If you push in and release on this button, you press Enter or confirm a setting when you are in any audio settings menu.
If the volume control isn't working, check if the mute is on. Press once to toggle the Mute. The mute button is just below and near the right side of the optical drive slot.
If you still can't change the volume, check if you have the wiring correct and no shorted wires. Look for damage at the speakers (or evidence of mice chewing on the wires). Also check your audio balance. See the manual page 12 here: http://www.voxxintlcorp.com/docs/common/VM9312/VM9312_OM.pdf
. You may also want to try a system reset. (Press and hold the reset button for a few seconds by using the tip of a ballpoint pen in the little opening at the lower right hand side of the faceplate.)
I hope this helps.