These barks, or other barks... The voice was granted to all creatures, including us, humans, for communication. He barks, unable to catch the ball, for his protest. For the same reason he barks when left alone. The trick is to make him bark solely because you ask him to do that and reward.
KZoppa provided you with a clue - you yourself should take the initiative to produce the sound. Not necessary the dogs, could be a bird, start to bark yourself at it first, before he opens his mouth. If he looks in the same direction as you do and starts barking - support your own bark with the verbal command in repetition. If you see something suddenly, indicate it to him with the command and support with your own bark. Your behaviour will sound for him the behaviour of the leader of the pack, who alarms the pack for some reason. Always reward him with a treat, because later on it will be the treat in your hand only to drive him. Then, this "Spe-e-ak" doesn't sound very exiting, becouse it starts with a sibilant and ends with a voiceless consonant. Simply "Bark!" will do. Use the command in German if you like, German sounds sharp. It could be better to change the sound of your command from now on anyway, as well as the drive.