This is an *exceptionally* rare bug, but I'm documenting it anyway. Under unknown circumstances, the game will confuse itself as to what sound it should play when a weapon is fired. When I've seen it, it's always been laser weapons suddenly swapping to using the recoilless cannon sound effect. I have seen this once in an older version, and once in 1.6b2. It is rather exceptionally rare, so it may not be replicable. On both occasions it has occurred in the middle of pitched combat. Weapon in the 1.6b2 incident was modded, but contained no exotic code.
Restarting the game will clear the problem. Unknown if a simple reload will fix it.