Hi, there. Usually what I do is just do a, 'stop all commands' to stop all playing sounds. There is no feature in VA to allow you to stop a particular sound, other than what the others have said about playing another sound that interrupts a currently-playing sound.
I looked at adding an action way back in the beginning to cancel a particular sound... You would have to have an action to cancel each and every sound, though (pain)... I would entertain the idea of making a cover action that cancels all sound only (and not stop already-running commands)... that may be something worthwhile.
Gary