Apple IMac won't play music through Iphone ear buds. Help?

The earphones work fine when I play music in the phone but when I insert them in to the computer they don't play, when I turn the volume up they just show a circle with a cross through it. My other Apple earbuds used to work fine in the earphone jack but I lost them and bought new ones. If there's any computer jack experts, or even some one who has previously had this happen before or know why my earbuds aren't playing when inserted in to the computer jack, please leave a comment below on how to fix it so they will play when plugged in to the computer it would be greatly appreciated!! Thanks! :)