First check system preferences>output tab and make sure the balance is set to center, Does the right speaker have sound with headphones? Then make sure your cable is good, get another to check and make sure. Take the cable end that connects to the speaker and swap them, so that what should be the left speaker is now actually connected to the right.
If after all this, you might want to consider a trip to the Apple store, or a call to Apple.
Best of luck