All stimuli do not generate reactions. It is the CNS that decides if a reaction has to be generated on receiving the information about a stimulus.
External stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response. In order for a stimulus to be detected with high probability, its level must exceed the absolute threshold; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.
The changes that occur inside ear,vibration of tympanum, movement of ear ossicles and all are responses to the stimulus (the mentioned two are mechanical responses) to transmit the information about the stimulus(sound) to the brain.
If the brain finds out that the sound is actually a noise then it would instruct the body (limbs for example) to move away from the source of the sound. This is then called the reaction.
If the sound is not that loud, i.e. not potentially damaging to the ears then the CNS does not instruct to retreat.