Here is more information for you.
As of R9.1, you have the following alternate options of Hold Music:
WAV: To specify a wav file, enter WAV: followed by the file name. Playback resumes from where it left off the last time.
XTN: Any analog extension. Not applicable to Linux systems.
WAVRST: To specify a wav file, enter WAVRST: followed by the file name. Playback is started every time from the beginning.
WAVDIR: Multiple WAV file source. The WAV files must be stored in the /opt/ipoffice/tones/mohwavdir/ directory. Playback resumes from where it left off the last time. Not applicable to IP500 V2 systems.
WAVDIRRST: Multiple WAV file source. The WAV files must be stored in the /opt/ipoffice/tones/mohwavdir/ directory. Playback is started every time from the beginning. Not applicable to IP500 V2 systems.
USB: Supports multiple USB inputs. Enter USB:<number>. Not applicable to IP500 V2 systems.
LINE: In Server Edition networks, setting the Secondary Server and Expansion Server Alternate Source to Line allows the server to receive streamed audio from a source on the Primary Server. On the Secondary Server and Expansion Server, enter Line:x,ywhere x is the line number to the Primary Server and y is the MOH source number on the Primary Server.