Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† phthongus, i, m., = φθόγγος, a sound, tone: Saturnum dicebat Dorico moveri phthongo, Jovem Phrygio, Plin. 2, 22, 20, § 84; Fulg. Myth. 3, 2 sq.; Mart. Cap. 9, § 939 (pure Lat. sonus).