I recently acquired two sets of sweet Dual Disruptor Heavy Cannon Mk VI for my federation heavy escort.
I got these from the PVP vendor on deep space K7.
They are very effective in combat, and really tear Birds of Prey a new one. But they fire completely silently. No sound at all when I fire them at a ship.
My quantum torpedoes make sound, my rear facing Disruptor beam makes sound. The only problem I am having is with these two heavy disruptor cannons. Every other single sound in the game works just fine.
I tried resetting my sound options to default, restarting the game, and restarting my computer. Nill.