Monica Farnsworth had it all: looks, money and notoriety. She longed for adventure. Being an accomplished singer, she took off on an Ocean Liner, working as a lounger singer. While sailing near the islands off Greece, she encountered one of the legendary Sirens. The siren had heard her singing, and was enthralled by her voice. In return, she gifted her with the power of the Sirens, charging her to use her new gifts to help, not harm, mankind.
Returning to America, Monica used her fortune to train in unarmed combat (as well as using Brass Knuckles), and various investigative techniques. She also trained herself to use her new found gifts, learning to adjust her pitch and tone to achieve various effects.
Once she completed all her training, she had a costume made, and soon The Scarlet Siren was prowling the streets, taking on the scum of the earth!
Note: She has one extra advantage, so I added a point to the final cost for her powers. Also, I charged 2 points for the Variable enhancement, as I know some cost 2 (and figured variable is one of them). If that's wrong, let me know, and I'll adjust accordingly. Hope you all like her!
Edit: updated with the corrections lindharin suggested.