Story
Is Sypha dead?
Yes, before this story starts
Sypha Belnades is dead when Castlevania: Belmont's Curse begins. She is Rose's mother. Twenty-three years earlier she and Trevor defeated Dracula. Konami's character page says she dies of Dracula's curse soon after Rose is born. Alejandra Reynoso voices her.
The game still shows her. GameSpot notes a moment in the Hall of Mirrors where Sypha speaks. That is a memory or an image, not a reversal of the death. A specific calendar year for the death, beyond 'soon after Rose's birth', is not on the character page, so it is not printed here.
What that death does to the plot
The curse that killed Sypha is the same curse in Rose's right arm. The family-curse page separates that from older series talk about the Belmonts being outcasts. Sypha's death is the family's wound at the start of this game, and it is stated by Konami, not inferred from a cutscene description.
What I ran into
People ask this because the series keeps bringing speakers back as magic. I put the answer in the title and the first sentence so nobody has to finish a essay to learn she is gone.
Questions
Is Sypha dead?
Yes. She dies of Dracula's curse soon after Rose is born, before the 1499 story.
Who voices Sypha?
Alejandra Reynoso, on Konami's character page.