In your solar system, something has caused a constant stream of debris to enter the atmosphere of your world. The result is a constant loud rumble or thunder, constantly changing in pitch and volume depending on the debris that enters the atmosphere.
Sound is nothing more than vibrations in the atmosphere. Or, more generally, vibrations in any medium. We humans call it "hearing" when the atmosphere vibrates and "feeling" when something else, like the floor, vibrates.
So the question simplifies to, "what can cause the atmosphere to vibrate enough to lead to limited or no evolution of the ability to hear?"
I'm voting for something entering the atmosphere constantly. As it enters, it causes explosions, rumbles, and thunder-sounding-stuff. If the mass varies from dense streams of dust or pebbles to larger Buick-sized objects, then you get variation in frequencies and volume.
An alternative (and it might even be easier to rationalize than dust/debris) is for energy such as the Solar Wind to cause constant lightning to form on the planet, leading to constant sound.
So, the next question is, what can cause debris to constantly enter the atmosphere over the time scales necessary for evolution?
The lightening might be easier to justify because you could do something like a binary star where one star is stealing from the companion star and the resulting spiral of stellar debris is something the planet must regularly (if not constantly) pass through. This is a fast and kinda tag:Science-Fiction type of answer, but frankly, we're deep in tag:Science-Fiction territory anyway.
So, let's focus on the debris. Planets have a habit of sweeping their orbits clear of debris fairly quickly when we're talking about evolutionary time scales. So this must be something that's constantly adding new material to the orbit of the planet. Conveniently, I shouldn't think that (compared to the mass of your planet) a large quantity of material would be needed to achieve the necessary sound.
Which is good, otherwise we'd have to deal with your planet's diameter increasing by a meter or two a year, which would have massive consequences on evolutionary time scales.
My suggestion is an asteroid field caused by the break-up of two super-Earths early in your world's evolutionary period. The assumption is that together they represent a LOT of material to draw from. The goal is to have a very wide asteroid field, one so wide that it places the edge of the field at the orbit of your world. As mass within the asteroid field bounces around, it's constantly (but not debilitatingly) bouncing into the orbit of your world.
Is this believable?
I'm comfortable with its ability to meet suspension-of-disbelief. From a scientific perspective, what I've described is impossible. Such an asteroid field would remain tightly bound due to the nature of gravity. It wouldn't spread out as I've suggested. And if it did, there's enough mass involved that it would, I believe, have a pretty good chance of slowly drawing your world into the asteroid field to eventually become the core of a new massive planet (maybe even a dwarf star, given the mass needed for the two original colliding worlds).
But, then again, if that were really happening, that would be a convenient way to rationalize the debris causing the noise. It just means your planet has a life span that's quite a bit shorter than Earth's.