The sound will really crack if you are using it at full volume in a long periods of time. If it still under warranty then try to have it checked into an authorized service center.
Too high volume leads to "basag" sound of your speaker especially when you take it too long because you enjoy the music. If you think that it needs a repair, don't hesitate to bring your phone to the service center to check if the speaker is damage or what not. It's always better to take good care of our phones and do not abuse using it.