I have the same problem. It has something to do with the phone thinking it's in headphone mode and not switching out. You can sometimes get it back to speaker by playing sound and plugging headphones in and out until it works, but it won't stay that way.
My phone continually flips back to that kind of silent mode. I've looked this up a bit and it appears it's been a problem for a long time.
Very disappointed with this product for this reason. I miss calls and texts because of it.
I've never even managed to get it back to normal by plugging headphones in and out. I've read about it and seen that are many people out there with a problem similar to this one that can solve it just by doing that.
Now, i would like to add some new info on this. Normally i don't get any sound from my iPhone, can't play musics nor hear any sound on apps or on a call. If i turn the volume up or down, i see the volume control saying that it is the Headphone's Volume even though there is no headphone plugged.
Today i woke up and when i unlocked my iPhone i noticed that it made that clicking sound. I was all happy believing that my iPhone was magically back! Listened to music, used an app and all was ok.
Then i tried to make a call. Even though i hear the sound of apps and music, and when i turn the volume it's no longer showing Headphone's Volume, i still don't hear anything when making a call!
People can hear me and even with the sound working with everything, i still can't hear nothing when making a call without the headphone's or on loudspeaker.
Does anyone have any idea if this can be a software problem? I'm thinking on doing a downgrade to see if things get back to normal.
If that doesn't work, i'll go through with the long and inefficient way out of taking my iPhone to my operator's support...