rough approach to making any CitrusSoundGroup monophonic (one sound at a time) by stopping all other sounds playing in the group before playing.
with _ce.sound.getGroup(CitrusSoundGroup.BGM).polyphonic = false;
only one sound can play in the bgm group at a time so you can just use playSound() and all previous sound will be stopped.
(but remember sound groups can be created and added at anytime you are not restricted by the default 3.)