Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
armĭ-sŏnus, a, um, adj. [arma-sono], resounding with arms (poet.): numina Palladis armisonae, Verg. A. 3, 544: antrum, Claud. Rapt. Pros. 3, 67.