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.