The answer is meant to be provided by an old man who gives a hint, quite near the room with Digdogger, the boss of Level 5.
Of course, you're likely to have suffered by these creatures quite a bit by then. Also, the North American release's hint isn't really completely clear: "Secret Power is said to be in the arrow". For a long time, I wondered if that was actually meant to also provide a hint on how to defeat Gannon (in an entirely different level). I'm rather doubting that now, since Legends of Localization: Zelda 1 Pols Voice section points out that the Japanese version's text translates to something like "There are some creatures that are weak against sound."
Speaking of that website, it includes a reference to a few videos on YouTube that demonstrate some various ways that the creature is weak in various releases of this game. Besides the demonstration of sound killing Pols Voice on the Famicom (a video that includes a yell), there is: