The Ultimate Inventory System shares the Audio System with the Ultimate Character Controller.

Please find the doucmentation for the Audio here

Specific to the Inventory system, you will find a simple yet very useful component.

Button Audio Handler

Add this component on the same gameobject as your Button, ActionButton (or ItemViewSlot) to play sounds.

Button Audio

It takes in a ButtonAudio object. To create one do right-click in the project window:
Create/Ultimate Inventory System/UI/Button Audio.

For AudioConfigs, refer to the rest of the doucmentation for the Audio here