how to Clean and Reinstall OS6 in iPhone 4S

I purchased iphone 4S last Dec.06,2012 on online store here in United Arab Emerates(SOUQ.COM).i found out that the internal speaker is not working when playing music and video,i guess there is software problem because when i used my headset i can head sound but when i remove the head set the sound not coming out on internal speaker.I try to use my Airport Express wi-fi and then i check the air play and i saw the airport express and the dock connector instead of iphone.I want to clean and reinstall the OS 6.Please help me!!!!!!