Control the volume of Volumio

I have set up a Raspberry Pi with the Volumio player. Now I want to control the volume with a rotary encoder. And also want to pause or play the current song.

#!/usr/bin/env python
#
# Raspberry Pi Rotary Test Encoder Class
#
# Author : Bob Rathbone
# Site   : http://ift.tt/1sHHWM2
#
# This class uses a standard rotary encoder with push switch
#

import sys
import time
from rotary_class import RotaryEncoder
import subprocess

# Define GPIO inputs
PIN_A = 21  
PIN_B = 16  
BUTTON = 4

# This is the event callback routine to handle events
def switch_event(event):
    if event == RotaryEncoder.CLOCKWISE:
        print "Volume up"
        subprocess.call(['mpc', 'volume', '+1'])

    elif event == RotaryEncoder.ANTICLOCKWISE:
        print "Volume down"
        subprocess.call(['mpc', 'volume', '-1' ])

    elif event == RotaryEncoder.BUTTONDOWN:
        print "Pause/Play"      

#   elif event == RotaryEncoder.BUTTONUP:
#       print "Button up"
    return

# Define the switch
rswitch = RotaryEncoder(PIN_A,PIN_B,BUTTON,switch_event)

while True:
    time.sleep(0.5)

This is the coder I already have. But when I start it, and try to set the volume +1, I just get an error.

This one:

Traceback (most recent call last):
  File "/home/pi/radio/rotary_class.py", line 87, in switch_event
    self.callback(event)
  File "./test_rotary_class.py", line 35, in switch_event
    subprocess.call(['mpc', 'volume', '-1' ])
  File "/usr/lib/python2.7/subprocess.py", line 493, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 679, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1259, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

It would be great if someone can help me and tell me how to do the pause/play :)