Go to the BP_TurretAIComponent then create a new variable called FireSound, set its variable type to Sound Base and tick Instance Editable.

Next add the highlighted nodes shown in the image below, location at the top of the screenshot.

Now in the class defaults you can set the fire sound for the turret. You can also select the BP_TurretAIComponent in your turret blueprint and edit the Fire Sound in the details panel.