Polycom MGC Administrator Administrator's Manual page 219

MGC Administrator's Guide
If the Greet and Guide option is added to the MCU, one Audio Module
per MCU is used to store the Greet and Guide messages and play
background music. To enable the Greet and Guide mode, two additional
extensions are installed on the Audio Module: the Audio Message
daughter card (Messages Extension) and the Music I/O card (Music
Extension).
Only one Music extension and Messages daughter card are installed per MCU.
Once these extensions are installed in the MCU, they are automatically
detected by the system at restart. The Message Extension and the Music
Extension check boxes are checked to indicate that these extensions are
installed in this Audio module and that they are enabled.
3. Click OK to exit the Card Settings dialog box.
4-37