Hai,
I had the same issue with my sisters i-phone, and i read all these forums.As one said i tried software updation but it cudnt help. IF its the prob, then speaker wudnt work at all like wen v play music No sound wud b der. But this case is different. The solution is very very Simple..
Go to the TOP LEFT SIDE OF UR PHONE THERE IS A BUTTON(MUTE BUTTON).. Turn it other way ..if u have a case, open the square shaped box of the case and do it.
The mute button may b turned on accidentlyso jus turn it off and try.