When you cast a sorcery spell, Spellweaver Volute's ability triggers. It will make a copy of the enchanted card. The copy will exist in the graveyard. You'll have the option to cast the copy. If you don't, the copy remains in the graveyard and ceases to exist the next time state-based actions are checked. If you do cast the copy, the copy is cast from the graveyard. After you finish casting it, the enchanted card is exiled and you must choose a new instant card in a graveyard to attach Spellweaver Volute to. If you can't, Spellweaver Volute remains on the battlefield attached to nothing and is then put into the graveyard the next time state-based actions are checked.

Say Spellweaver Volute's ability triggers, then Spellweaver Volute leaves the battlefield in response. Then the instant card that was enchanted is removed from the graveyard in response. When the Volute's ability resolves, it will check its last existence on the battlefield and identify the "enchanted card" as that instant card, so the card is copied and controller of the triggered ability may cast it.

The Aura will be on the battlefield attached to a card in a different zone. The Aura won't be in the graveyard. The enchanted card won't be on the battlefield.

Say Spellweaver Volute's ability triggers, then the enchanted instant card is exiled in response. Spellweaver Volute, which is now enchanting nothing, is put into its owner's graveyard as a state-based action. When the Volute's ability resolves, it will check its last existence on the battlefield and identify the "enchanted card" as "nothing," so no copy is created.

This Aura targets, and enchants, an instant card in a graveyard. This is the first Aura that enchants something that's neither a permanent nor a player.

If the enchanted card changes zones (due to being cast with flashback, or being exiled with Cremate, for example), Spellweaver Volute "falls off" and is put into its owner's graveyard as a state-based action.