This is a short guide on how to change the AIs hearing distance, this controls how far the AI can hear. Start by going to the BP_MasterAIController then in the components tab select the AIPerception. Then click the Senses Config drop down option then the 1 drop down option then the Sense Drop down option and you should see the options shown below.

Hearing range is the distance the AI can hear in CM so 4000 is 40 meters. You can increase or decrease as you need.

If you want to know how to change the Sight Distances check out the guide linked below.