Go to the BP_PlayerInventory and create a new variable called OpenInventorySound and set its type to SoundBase and tick on Instance Editable.

Create another variable called CloseInventorySound and set its type to SoundBase and tick on Instance Editable.

Next add the highlighted nodes shown below, locations at the top of the images.

You can now set the open and close sounds in the class defaults of the BP_PlayerInventory.