How to make a mob make a sound when it sees you

You're going to have to do something using Raytrace procedures. My best bet would be to make a procedure that triggers on Entity Update Tick, checks if it's your custom entity, then checks if the entity's look angle position is within a certain range of the player's location. You might also want to add some sort of cooldown variable so it doesn't constantly make the noise when the player is in its range of sight.