Base effect: All allies within 30 feet gain a +5 morale bonus to attack rolls, damage rolls, skill and ability checks, and saving throws.
Your song about hope reinforces belief so much that it can affect reality. All allies can re-roll a single d20 roll made before the end of their next turn.
Weak Refrain: the target re-rolls the d20 before knowing whether the result was a success or a failure.
Intermediate Refrain: the target re-rolls the d20, adding their Charisma bonus (minimum +3) to the result, before knowing whether the result was a success of a failure.
Strong refrain: the target re-rolls the d20 after knowing if the result was a success or a failure. Unless they roll 1 on this re-roll, the outcome of the roll is a success (with minimum result to achieve success, should the actual result of the roll matter).