If you want to stick with the "sonic canister" concept, without it becoming a hard-shelled missile, how's this: the gun produces the sonic vibration, encapsulates it in a magnetic-field (or something similar) 'bubble' and fires the bubble-missile at the enemy. When the magnetic field impacts on the enemy's hull/shields, it bursts, channelling all of the vibration into a single spot, like a shaped-charge explosive. The magnetic field would replace any limited onboard 'ammo' supply of canisters to carry the sound...