Has your son always been that way? That is, could he possibly have a hearing problem and not be able to hear your voice when you speak normally?
But often a child listens only to Mama's raised voice because he knows then than she means business. There's an old story about Jimmy playing in the back yard with his friend Billy. Jimmy's mother calls from the back door, "Jimmy, come here," and Jimmy keeps on playing. Billy says, "Didn't you hear your mom call?" Jimmy says, "Yes, but I don't have to go yet." A minute later his mother calls, "Jimmy, I want you right now!" Jimmy says to his friend, "I don't have to go yet." A couple of minutes later they hear, "JAMES EDWARD, DO I HAVE TO COME OUT THERE?" Jimmy says, "Now I have to go!"
So if your son is not hard of hearing but only hard of listening, you could try this. It can't hurt. "Jimmy, I'm going to play a game with you, because I don't like yelling at you. Any time you hear me call your name, I want you to turn around and look at me. That way I'll know you heard me and are ready to listen. So keep your ears tuned in. It could happen any time." Then be sure to call his name frequently. Do it when he's busy, and when he's doing nothing much. Call in your usual voice. When he's nearby, call in a whisper. Use a funny voice once in a while just to make the whole thing more interesting. When he turns to face you, commend him. Attention reinforces the behavior you want to develop. Maybe if he listens when he's called, say, twenty times in the next couple of days (not necessarily twenty times in a row), you might decide you both deserve a reward - him because he's being attentive, and you because you're not yelling.
Do this for several days. Be sure to commend him when he is attentive. You can let him know what a good thing it is to have ears turned on and ready to listen. You might even read some books to him in a *very* quiet voice. Tell him you're practicing not yelling. Actually, you're helping him to practice listening to your voice.
I am sort of a quiet person myself, but I remember doing a lot of yelling at my children to, um, emphasize points. I now wish I could have that time back to do it again - better and more quietly. I wish I had saved raising my voice for if the house were on fire.