It's been a while since I touched one but I believe there is a 3.5mm jack on the top near where the cables plug in to the cards. This will be the first MOH input. You can have others if you have additional resource cards.
Because this is the first input it should just start playing to all callers put on hold. It should also play as BGM (BackGround Music) if you dial a code on the phone.
If you had additional MOH sources then you could specify them as music sources for specific xtns, specific HUNT groups, or as BGM (to differ from MOH, eg have Music0 as MOH and Music1 as BGM).