The New Hampshire man charged with killing a University of New Hampshire sophomore a year ago has waived arraignment on new charges that he conspired to mislead investigators. Seth Mazzaglia, 30, was charged last year with first-degree murder in the disappearance and death of Elizabeth ‘‘Lizzi’’ Marriott of Westborough, Mass. Last month, he was indicted on charges of conspiracy to hinder the prosecution. He faced arraignment by video Thursday in Strafford Superior Court but waived arraignment. Prosecutors said Mazzaglia discussed framing and killing a witness in letters to his former girlfriend. They also say he sent fraudulent text messages to mask Marriott’s whereabouts.