A person's words are rarely just empty sounds; they act as a window into the deepest parts of their mind. In the case of Job, his arguments are not viewed as the spontaneous cries of a suffering man, but rather as calculated expressions of deeply rooted sin and heresy that have taken root over time.
The act of speaking here is understood as a process of teaching, habituation, and guidance [מצודת ציון, מלבי״ם]. Commentators split into two main ways of understanding how this process works. The first approach focuses on an internal struggle. According to this view, the sin itself, whether it is an evil urge, a corrupt heart, or a distorted mind, actually trains the mouth to speak against God [רש״י, מלבי״ם, ביאור שטיינזלץ, אלשיך, תקות אנוש]. One wrong step leads to another, and sinful thoughts eventually turn into fluent speech. This fluency proves that Job had been turning these heretical ideas over in his mind for many days [מלבי״ם, אלשיך].
On the other hand, a second approach sees this process moving outward. Instead of the sin teaching the mouth, Job's mouth actively broadcasts his sin to the world. By speaking out, he exposes his inner wickedness and creates a dangerous situation where others might learn to follow his destructive path [מצודת דוד, רמב״ן, אבן עזרא].
The manner in which these words are delivered is just as important as the message itself. The primary approach among commentators is that Job does not hurl his heresy at God openly. Instead, he carefully wraps his arguments in thoughtful, sophisticated, and crafty language [מצודת דוד, מלבי״ם, ביאור שטיינזלץ, רמב״ן, אלשיך]. He presents himself as someone merely asking innocent questions about reward and punishment, but this is actually a clever trick designed to hide his true intentions [רמב״ן, מצודת דוד]. The very use of such elevated, intellectual speech proves that his words are the result of careful thought and hard work, rather than an accidental slip of the tongue from someone in pain [מלבי״ם].
While most see this sophisticated speech as a description of what Job is actively doing, a minority view reads it as a sharp critique of what he should have done instead. From this perspective, rather than allowing sin to speak freely from his throat, Job should have chosen to use the language of the wise [רש״י].