The stopsound command stops the sound with a delay on the client side which causes sounds to start playing and than stopping even if the command is run directly after the sound is played.
Example: the xp command is used to increase the players xp in a function. The level increases from 4 to 5 which causes the level up sound to play. Directly after the stopsound command is run but the sound is not correctly stopped and can be heard for a short period of time.