I need to know how to adjust the volume on a Merlin Plus 820. The volume on the system's moh port needs to be louder.

1st off, keep in mind that system uses an 8 ohm input unlike the standard or usual 600 ohm input that most systems use.
Did you try adjusting the output of the sorce?
Go to the administrator/ attendant voice terminal and slide the T/P switch to P and press administer.
*12 is off * 13 is low * 14 is medium
and *15 is high. then press administer again,
it should beep twice. Slide the T/P switch to the center.

You can also use it as Background music buy using the steps above with the codes, *16 = off, *17 = Low, *18 = Med. *19 = high