Here, Mort Walker plays with established comics language – or perhaps established comics semiotics, if we want to be more precise and pretentious. Beetle and Killer decide to try to fool Sarge into believing that he's losing his hearing, so they "talk" soundlessly.
Sarge responds equally soundlessly in panel 5 – but Walker uses enough established comics symbols for "loud yelling" to convince them – and us readers – that his response is, in fact, louder than loud. And yet, he only needs to use two (or perhaps four) symbols for "loud yelling" to give us that impression – in spades.
First, there's Sarge's face. He a) looks angry and b) his mouth is open wide. Hence, we "know" that he is yelling angrily. Second, there's his word balloon. It is c) drawn with a heavy outline, which is an established symbol for loudness – perhaps the outline needs to be heavier and stronger to be able to contain all the loud words in big, fat letters? – and it is also d) very large, which is necessary to make room for all those big, fat letters.
Two more things support the impression of loudness given to the reader. In the preceding panel, the scowling look on Sarge's face indicates that he is both on to the soldiers' scheme and not pleased about it, and in panel 5, the hasty exit by Killer and Beetle supports the notion that Sarge is now so angry that the smart thing is to not stick around – and one indication of how angry somebody is is of course how loudly they are yelling at you.
Brilliantly done, and very funny.