Song of the Sirens was an enchantment given to Hercules by Circe for him to use as a way to reach Medusa affectionately. When this jar was opened, the voices of sirens would sing, allowing for an atmosphere of love to be set. Medusa immediately lost control of her mind and was drawn to Hercules easily.
It was revealed later, that the enchantment had some consequences, and Medusa fell very ill with a terrible infection growing across her body. Hercules believed it to be a trick by Circe. (The Song of the Sirens)