By default, the first caller in your conference will hear hold music, regardless of whether they dial in with the Participant Access Code or Moderator PIN. The music will stop once two callers have joined your conference, either by phone or via their computer using the Online Meeting Room.
You can disable the hold music from the Settings page, under Chimes & Name Announce. Just toggle 'Waiting room music' to the OFF position and you’ll hear beeps rather than hold music until the second party arrives.
It is not currently possible to change the hold music.