The acoustic behaviour of (NH4)(4)H-2(SeO4)(3) single crystal around the superionic phase transition at 378 K was studied by Brillouin scattering measurements. No anomaly of the longitudinal mode in the [100] direction was observed whereas 5% softening of the longitudinal mode with a minimum at 382 K was found in the [001] direction. The possible symmetry of the crystal above the superionic phase transition is discussed.