how do i make an item if you hit a mob then it will give a custom sound

Hopefully this can help a bit;

you will have to make a procedure that checks if an entity is attacked
then check if the source entity has the item you want to have the sound played
then all you have to do is put the sound block into the procedure

Event Trigger = Entity Attacked

If (item in main hand of) (Source Entity) = (CUSTOM.ITEM)
(Play at xyz level: 1 pitch: 1 category: --- sound: (Insert Sound You Want Here))