You know, that might actually work as a serious Power.
Work, yes; serious, no. Dresden Files does not need a musical episode.
Disney Music Number (-2): You can make a performance roll to sing an epic song, anyone else in the scene has to roll discipline or they too must start singing, singing counts as a supplemental action (-1) and all actions in a Disney Music number are modified by performance.
Suggested 'fix':
Disney Music Number (-2): At the start of scene (or with an action) you may spend a Fate Point and make a Performance roll. Anyone who can hear or see may make a Discipline roll to resist. Those who fail immediately join the song and dance. Their actions are restricted by Performance for the remainder of the scene. You also gain a free tag of the scene Aspect PERFECT CHOREOGRAPHY which may be used to compel individuals to copy each other's actions (subject to all the normal rules for compels).
Pun-hit Wonder [-4]
* You may perform any form of magical effect imaginable (using the relevant rules; with Performance instead of Discipline), provided a pun is somehow involved. However, every additional time you make the same pun (for as long as the other players can remember) your magic rolls are at a cumulative -1. After all, a pun used to often isn't exactly punny anymore. You also receive 8 potion slots, which must be items that are in some way a pun.
* +2 to Rapport rolls when dealing with old-world types.
* You gain a +2 to the Provocation trapping and may use it with Performance instead of Intimidation.