When gentle guidance and logical appeals fail to inspire change, a shift in strategy becomes necessary. God turns to strict, frightening warnings of approaching disaster, hoping that fear might finally motivate the people to repent and correct their ways [מלבי״ם]. Yet, even these urgent alarms are met with absolute stubbornness and a complete refusal to listen.
God had already placed protectors over the people in the past, much like guards stationed on a high tower during times of war [רש״י, רד״ק, מצודת ציון, שטיינזלץ]. The primary approach among commentators is that these watchmen represent the true prophets. A physical guard's duty is to look out over the landscape and, upon seeing an enemy army approaching, blow a horn to warn the citizens to arm themselves and rush to the city walls [רש״י, מצודת דוד, מלבי״ם]. Similarly, the prophets serve as spiritual lookouts. Their role is to sound the alarm about impending destruction and urge the people to fix their behavior before it is too late.
The demand to pay attention to the sound of the horn is a plea to listen closely to these moral warnings, which are designed to awaken a healthy fear of punishment [רש״י, מצודת ציון, מלבי״ם, שטיינזלץ]. Alternatively, this call serves as a harsh reality check. The true prophets are urging the people to stop believing the empty promises of fake peace offered by false prophets. Instead, they must face the truth and hear the actual sounds of the enemy already gathering around them [רד״ק].
Despite the clear and present danger, the people flatly reject the warning. They have no interest in finding the right path or accepting any moral correction [מצודת דוד, שטיינזלץ]. This stubborn refusal stems from a deep sense of complacency. They dismiss the urgent alarms because they hold a mistaken belief that simply living in Jerusalem guarantees them absolute peace and security [רד״ק].