If you have an iPhone 4 or 4S, try to play any music or sound, then use finger or thumb to block the speaker hole located at the bottom of your iPhone, then you will find only one speaker will work, the right speaker side. not work for the left side.
Only one speaker works, is it problem with your iPhone?
The answer is not problem, because although the shape and looks the same as speaker, located at the left side is not a speaker, it is a microphone.
Do not think your iPhone is in the trouble because the iPhone 4 / 4S only have one speaker is located on the right side, while the left is the mic/microphone.