How do we hear ant sounds in Ant-Man?

The movie focuses on ant sounds to highlight the action they want followed. Selective sound highlighting is typical in all movies. Like how you can clearly hear people in a club talking at normal conversation levels which would be impossible in real life.

That said, we typically hear the ant sounds only when the camera is in ant perspective, just as we do the vision changes. We, the audience see things similar to how shrunken Scott sees them, and hears them. Perspective is distorted, when needed. The camera, being closer to the ants, making them the relative size of a helicopter, will sound different then when the camera makes the ants the relative size of an ant. Comically, this is illustrated when YellowJacket is struck by a train:

Notice the sound changes? That's the perspective change. Also an example of the Doppler Effect.

In reality, someone with ear drums the size of a fraction of an inch will hear the same sound differently from a normal person. Compare listening to ear buds, tiny speakers that need to be close to your ear drums for you to hear them properly, to listening to a concert speaker. The pressure wave of the sound is the same, but the intensities are much different.

In an earlier scene from the movie, when Scott first shrinks down, notice how Luis' footsteps sound like booms to Ant Scott, while Scott's yelling sounds like faint scratching to normal size Luis.

Of course, the following scene where Scott falls into the club, he should be completely disoriented by the loud club music. Not only does the suit helmet likely help prevent that, the Pym Particles allow Scott to keep his normal size strength, but mainly it's because the movie (and comics) do sacrifice scientific reality for entertainment reasons. The Rule of Cool applies more than fact would. And a story like Ant-Man would be ruined by facts.