Broadening of the highly localized nitrogen vibrational mode in the dilute nitride random alloy GaNAs has been calculated, including N-N interactions up to fourth-nearest neighbor that have been obtained from density-functional theory supercell calculations. The phonon spectra of the high-frequency modes associated with substitutional nitrogen atoms in the random alloy are calculated from a dynamical matrix constructed by a linear combination of modes localized near each nitrogen atom. As nitrogen concentration is increased, the main localized vibrational mode peak at 465 cm(-1) remains sharp but its intensity grows sublinearly with nitrogen concentration, while a weak diffuse background grows in the frequency range of 400-500 cm(-1). Characteristic vibrational frequencies associated with N-N split interstitials (at 419, 622, and 1081 cm(-1)) and with N-As split interstitials (at 693 and 734 cm(-1)) are also calculated.